Port https://github.com/pygments/pygments.github.io/pull/4/files

* 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maintained (the Iodide project is no longer maintained).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This action is obsolete, as building Pyodide is now done using `make pyodide`.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

For large files it's better to have a viewport because
then you can view both the form as well as the part of
the output you are interested in at the same time.

* "Try out Pygments!" is more descriptive than "Try it out!"
* Moved the description of the implementation to the end.
* Added a <noscript> tag suggesting `pip install` and pygmentize.
* Added a <label> for the textarea.
* Disable spell checking for the textarea.
* Reduced the scope of the reset button to the file input.
* Added autofocus for the language select.