Generate the docset index and pages with Python (#5799)

Replace the hand-maintained docs/docset/docSet.sql and the bash/sed
Makefile recipe with docs/docset/generate_docset.py:

- The search index is generated from the mkdocs.yml nav and each page's
  H1 and declaration. Pages documenting several entities (e.g.
  JSON_HAS_CPP_11..26) get one entry per name; all non-API pages become
  guides. New API pages no longer need a manual entry.
- Page titles are set from the index names.
- The docset is self-contained: the mermaid loader no longer points to
  https://json.nlohmann.me/assets/..., the repository widget no longer
  queries api.github.com, remaining remote images are downloaded, and
  the build fails if any remote resource load remains.
- The CSS that hides the site navigation now uses Material's classes;
  the element selectors lost against them, so the header was shown.

check_structure.py's docset check is replaced by running the generator
in `make style_check`.

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

.github/CONTRIBUTING.md

@@ -149,8 +149,10 @@ make build -C docs/mkdocs          # strict build: fails on broken links, anchor
 make check_mermaid -C docs/mkdocs  # checks the Mermaid diagrams (requires Node.js)
 ```
 
-A new API page also needs an entry in [`docs/docset/docSet.sql`](https://github.com/nlohmann/json/blob/develop/docs/docset/docSet.sql),
-the search index of the docset; `make build` reports missing entries.
+The search index of the docset is generated from [`mkdocs.yml`](https://github.com/nlohmann/json/blob/develop/docs/mkdocs/mkdocs.yml)
+and each page's title (H1) and declaration by
+[`docs/docset/generate_docset.py`](https://github.com/nlohmann/json/blob/develop/docs/docset/generate_docset.py);
+`make build` reports API pages that cannot be classified.
 
 ### Amalgamate the source code