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/docset/README.md

@@ -4,7 +4,8 @@ The folder contains the required files to create a [docset](https://kapeli.com/d
 documentation browsers like [Dash](https://kapeli.com/dash), [Velocity](https://velocity.silverlakesoftware.com), or
 [Zeal](https://zealdocs.org).
 
-The docset can be created with
+The docset (pages and search index) is generated by `generate_docset.py` from the mkdocs site and `mkdocs.yml`. It
+can be created with
 
 ```sh
 make JSON_for_Modern_C++.docset
@@ -12,6 +13,7 @@ make JSON_for_Modern_C++.docset
 
 The generated folder `JSON_for_Modern_C++.docset` can then be opened in the documentation browser. `make all` builds a
 `JSON_for_Modern_C++.tgz` archive instead, and `make install_docset_zeal` installs the docset for Zeal directly.
+`make docSet.dsidx` builds only the search index.
 
 A recent version is also part of the [Dash user contributions](https://github.com/Kapeli/Dash-User-Contributions/tree/master/docsets/JSON_for_Modern_C%2B%2B).