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>

docs/mkdocs/Makefile

@@ -17,6 +17,7 @@ build: style_check
 
 style_check:
 	@cd docs ; ../venv/bin/python3 ../scripts/check_structure.py
+	@venv/bin/python3 ../docset/generate_docset.py list > /dev/null
 
 # check that all Mermaid diagrams parse (needs Node.js)
 # This target is used in the CI (ci_test_documentation_mermaid).