* Move the generation into sphinext, similar to other overviews we have.
* Generate a table instead of the plain list. This table contains links
  to the lexer class now.

* Support building docs when some plugins are installed

Pass plugins=False to get_all_lexers, otherwise we try to document
them, which fail. For styles, use STYLE_MAP directly; given that I'm
trying to introduce a new plugin interface, I'd rather not change
get_all_styles to support plugins=False right now.

* Add comment (and a fix)

* Add Make target for building Pyodide with currently checked out Pygments

Add a target to the documentation's Makefile to make Pyodide with
Pygments from the current checkout.

Additionally, use this target when building the documentation with
the demo page.

The new target works by building Pyodide on a Docker container, and
then exporting the build artifacts back to the host.

The Dockerfile used by the new target is based on the Dockerfile which
was used for building Pyodide on the CI:
https://github.com/pygments/pyodide-artifacts/blob/master/container/Dockerfile

The main difference between the two is that the new Dockerfile uses the
newest Pyodide base image, which is built by a repository which is actively
maintained (the Iodide project is no longer maintained).

The purpose of this change is to allow running the demo locally,
which was not possible previously, as Pyodide was only built by the
CI when deploying to Github Pages.

* Add instructions on how to run the demo locally to the README

* Update demo to work with Pyodide v0.20

Remove usage of Pyodide functionality which was deprecated in Pyodide v0.17:

- Loading Pyodide using `languagePluginURL` and `languagePluginLoader`.
- Access to globals via `pyodide.globals.x` has changed to `pyodide.globals.get("x")`.

Source:
https://pyodide.org/en/stable/project/release-notes/v0.17.0.html?highlight=languageplugin#api-changes

* Fix #2137

* Fix use of `styles` variable before it's defined

The `styles` variable was being used before it's value was set (which
is done when Pyodide finishes loading).

* Remove GitHub action for building Pyodide

This action is obsolete, as building Pyodide is now done using `make pyodide`.

* Upgrade Pages deployment action version

* Add to .dockerignore all files in .gitignore

* Change `pyodide` target to be a phony target

This commit adds a new url field to a lexer, which can be used to link
to the language website, instead of relying on having the link in either
languages.rst or the docstring of the lexer. Additionally, it changes the
languages.rst file to auto-generate the list of lexers from the actual
source code, using the provided URL.

Previously the form could only be filled out after Pyodide finished
loading (since the <select> options were generated via Pyodide).

This commit changes this, so that the <select> options are already
in the static HTML. The "Loading Python..." form overlay is removed,
instead just the Highlight button is disabled, letting users fill
out the form while Pyodide is loading.

When a ?code query parameter is given a "Loading Python..." message
is displayed below the form, since in that case users probably want
to wait for the highlighted code (instead of filling out the form).

* LilyPond: update builtins to 2.23.4

* Doc: deduplicate LilyPond in language lists

* Doc: exclude lilypond style from gallery

Because it's specific to LilyPond code and doesn't give good results
on Python code.