docs: new design for the documentation site (#5934)

* docs: new design for the documentation site

Apply the brand to the Material for MkDocs site: Inter and JetBrains Mono
served by the site (upright and real italic), brand colours, blue header band
with the syntax pattern, wordmark in the header, new home page (hero, cards,
animated feature tour), 404 page, polish for tabs, search, footer, focus
rings, tables and code blocks, and theme features (code copy, previous/next
footer, TOC follow, search suggestions).

Draw the number railroad diagram, the parser callback events and the iterator
illustrations as inline SVGs that follow the light and dark scheme. The
generator scripts are in docs/mkdocs/scripts/figures; the feature slideshow
(json-light.svg, json-dark.svg, replacing json.gif on the home page) is built
by docs/mkdocs/scripts/slideshow. Decision nodes in five flowcharts become
hexagons, which need far less room.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: add figures, table captions and a macro chooser

- Figures with numbered captions: SAX stepper, json_view layout and edit
  stepper, exception map, JSON pointer and patch walkers, QA pipeline,
  scaling and binary size charts, roadmap lanes, release timeline,
  integration decision tree and namespace builder
- Table captions (nj-table-caption) on 9 pages
- Compile times: modules and PCH charts, filesystem section, re-measured
  Figure 1, links between features/modules.md and compile_times.md
- Interactive macro chooser for features/arbitrary_types.md
- Link check: exclude URLs that answer the plugin with 400/401/419 or are
  dead (customers.md)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: macro chooser generates adl_serializer for non-default-constructible types

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: cross-link concept pages and API pages in See also sections

Add See also links in both directions between the concept pages (features,
home, integration) and the API pages, write API links in code font, give every
entry a comment after a dash, and show the callback event figure on the parser
callbacks page.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: address review comments on the new design

- Home: add a "Latest release" button
- Sponsors: update the list and group it by tier
- Features: list all feature pages on the index page
- Releases: add highlights per release, link the 4.0 roadmap, add the first commit to the timeline
- Exceptions: prefix all example messages with the exception name
- API: no code font for format names in the See also links of the to_/from_ pages
- Merge Patch: show the document before, the patch, and the result
- Replace "--" with typographic dashes, mark external links with an icon, center Mermaid diagrams

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_document/erase.md

@@ -54,7 +54,7 @@ has `erase`; calling it on a read-only `basic_json_document` fails to compile (`
 
 ## Exceptions
 
-1. Throws [`type_error.307`](../../home/exceptions.md#jsonexceptiontype_error307) if `object` is not an object -- the
+1. Throws [`type_error.307`](../../home/exceptions.md#jsonexceptiontype_error307) if `object` is not an object — the
    same message [`BasicJsonType::erase`](../basic_json/erase.md) throws for the same type.
 2. Throws `type_error.307` if `array` is not an array. Throws
    [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if `idx` is negative, or if
@@ -86,12 +86,12 @@ always starts from this document's own [`root()`](root.md).
 
 !!! info "Duplicate keys"
 
-    Overload 1. removes *every* member with `key`, not just the first -- unlike [`set`](set.md), which assigns the
+    Overload 1. removes *every* member with `key`, not just the first — unlike [`set`](set.md), which assigns the
     first occurrence and drops the rest. This is why it returns a count rather than a single view: there may be
     more than one member removed, or none.
 
 Like [`set`](set.md) and [`push_back`](push_back.md), `erase` never moves an element's *value*: a view still
-referring to a removed member or element keeps showing what it last held (see [Edits](index.md#edits)) -- it just no
+referring to a removed member or element keeps showing what it last held (see [Edits](index.md#edits)) — it just no
 longer appears when `array`/`object` is read, dumped, or iterated. What does change is the container's sequence of
 *links* to its elements: removing an element of `array` (2.) shifts the links to the elements after it by one, as
 [`insert`](insert.md) does in the other direction. *Every* iterator already taken over `array`/`object` is invalidated
@@ -101,8 +101,8 @@ by an erase, not only those at or after the removed element, since the container
 
 ??? example
 
-    The example below drops a deprecated field and a decommissioned entry from a configuration document -- using all
-    three overloads -- and shows what stays intact that would not with a plain `json`/`ordered_json` value: the order
+    The example below drops a deprecated field and a decommissioned entry from a configuration document — using all
+    three overloads — and shows what stays intact that would not with a plain `json`/`ordered_json` value: the order
     of the fields around the ones removed, and the exact spelling of a number that was never touched.
 
     ```cpp
@@ -117,12 +117,13 @@ by an erase, not only those at or after the removed element, since the container
 
 ## See also
 
-- [insert](insert.md) - insert an element into an array
-- [set](set.md) - replace a value, or set an object member, an array element, or the value a JSON pointer refers to
-- [push_back](push_back.md) - append to an array
-- [root](root.md) - the view of the root value, the starting point of overload 3
+- [`insert`](insert.md) - insert an element into an array
+- [`set`](set.md) - replace a value, or set an object member, an array element, or the value a JSON pointer refers to
+- [`push_back`](push_back.md) - append to an array
+- [`root`](root.md) - the view of the root value, the starting point of overload 3
 - [`BasicJsonType::erase`](../basic_json/erase.md) - the corresponding function of `basic_json`
 - [Edits](index.md#edits) - what an edit guarantees, for every overload
+- [Zero-copy JSON views](../../features/json_view.md#editing-a-document) - what an edit guarantees and costs
 
 ## Version history