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/features/parsing/large_and_partial_input.md

@@ -17,6 +17,8 @@ flowchart TD
     H -->|"yes"| I["stop early with SAX, or read through a view"]
 ```
 
+<p class="nj-caption" markdown>Questions about the input and the approach this page describes for each; an input can match more than one question.</p>
+
 ## Several values in one stream
 
 For newline-delimited input, read line by line and parse each line, as described in [JSON Lines](json_lines.md). For
@@ -80,6 +82,14 @@ exception is one past the buffer size in the first case:
 The example feeds a document in five chunks. The first four are reported as incomplete, and a syntax error is reported as
 such, not as missing data.
 
+The chunks of the example, and the bytes each attempt parses again:
+
+<div class="nj-fig-wrap" markdown>
+--8<-- "images/chunk_buffering.svg"
+</div>
+
+<p class="nj-caption" markdown>Each row is one parse attempt on the growing buffer: an error at the byte after the end of the buffer means the value is incomplete, an error before the end is a real syntax error. The last line adds up the bytes parsed over all attempts.</p>
+
 !!! warning "Limits of buffering"
 
     - The buffer must contain exactly one value. Data that belongs to the next value is a syntax error. Use framing if
@@ -93,6 +103,14 @@ such, not as missing data.
 
 The input does not have to become one huge `json` value. The two tools below process it piece by piece.
 
+How the memory use develops while a large array of records is parsed (illustrative, not a measurement):
+
+<div class="nj-fig-wrap" markdown>
+--8<-- "images/memory_over_time.svg"
+</div>
+
+<p class="nj-caption" markdown>Memory use over the number of records parsed: `parse()` keeps the whole tree, `json_document` the text plus its index, a discarding callback one record at a time, and SAX only what the handler keeps.</p>
+
 ### Handle and discard elements with a callback
 
 A [parser callback](parser_callbacks.md) is called for every finished value. If it returns `#!cpp false`, the value is
@@ -152,3 +170,8 @@ and gives read-only [views](../../api/basic_json_view/index.md) of the parts you
 - [SAX interface](sax_interface.md) - event-based parsing
 - [Zero-copy JSON views](../json_view.md) - read parts of a large document without building a tree
 - [Untrusted input](untrusted_input.md) - limits for input you do not control
+- [`operator>>`](../../api/operator_gtgt.md) - deserialize from stream
+- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
+- [`sax_parse`](../../api/basic_json/sax_parse.md) - generate SAX events
+- [`JSON_PRECISE_STREAM_POSITION`](../../api/macros/json_precise_stream_position.md) - leave the stream positioned right after a parsed number
+- [`JSON_DISABLE_STREAM_BULK_SCAN`](../../api/macros/json_disable_stream_bulk_scan.md) - read a `std::istream` one character at a time