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/mkdocs/mkdocs.yml

@@ -1,5 +1,6 @@
 # Project information
 site_name: JSON for Modern C++
+site_description: JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type.
 site_author: Niels Lohmann
 site_url: https://json.nlohmann.me/
 
@@ -14,33 +15,38 @@ copyright: Copyright © 2013-2026 Niels Lohmann
 # Configuration
 theme:
   name: material
+  custom_dir: overrides
   language: en
   palette:
     - media: '(prefers-color-scheme: light)'
       scheme: default
-      primary: indigo
-      accent: indigo
+      primary: custom
+      accent: custom
       toggle:
         icon: material/brightness-7
         name: Switch to dark mode
     - media: '(prefers-color-scheme: dark)'
       scheme: slate
-      primary: indigo
-      accent: indigo
+      primary: custom
+      accent: custom
       toggle:
         icon: material/brightness-4
         name: Switch to light mode
   icon:
     repo: fontawesome/brands/github
-  font:
-    text: Roboto
-    code: JetBrains Mono
+  font: false
+  logo: images/wordmark-header.svg
   features:
     - navigation.instant
     - navigation.tracking
     - navigation.tabs
     - navigation.indexes
     - navigation.top
+    - navigation.footer
+    - toc.follow
+    - search.suggest
+    - search.highlight
+    - content.code.copy
     - content.tabs.link
     - content.action.edit
     - content.action.view
@@ -619,3 +625,7 @@ plugins:
 
 extra_css:
   - css/custom.css
+  - css/brand.css
+
+extra_javascript:
+  - js/mermaid-brand.js