docs: add a version switch.

[miyukoc]

This patch makes the theme include a JavaScript file that replaces the static version HTML with a <select> element. The list of versions is fetched from <content root>/../versions.json and is expected to adhere to the following format:

[
  { "name": "1.2.3", "root_url": "/docs/amaranth/v1.2.3/" },
  { "name": "1.2.2", "root_url": "/docs/amaranth/v1.2.2/" }
]

This file is expected to be updated manually similarly to how updates are done for https://amaranth-lang.org/index.html. However, an automation could be built at a maintainer's request: for example, this file could be assembled anew when pushing from this repo's GitHub Actions workflow to https://github.com/amaranth-lang/amaranth-lang.github.io.

The appearance of the version switch matches that of the Read the Docs theme's version switch.

Once this PR is merged, a follow-up PR should be created for https://github.com/amaranth-lang/amaranth-lang.github.io that adds this JavaScript file to all past versions of the documentation, together with the initial version of the versions.json file.

[whitequark]

However, an automation could be built at a maintainer's request: for example, this file could be assembled anew when pushing from this repo's GitHub Actions workflow to https://github.com/amaranth-lang/amaranth-lang.github.io.

Yes please.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.28%. Comparing base (638edff) to head (0765235).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1642   +/-   ##
=======================================
  Coverage   91.28%   91.28%           
=======================================
  Files          44       44           
  Lines       11471    11471           
  Branches     2236     2236           
=======================================
  Hits        10471    10471           
  Misses        837      837           
  Partials      163      163           

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[whitequark]

The smoke tests can be ignored (amaranth-soc is having issues due to too-new Python). The check-links test might need an exemption for medium.com.

[miyukoc]

However, an automation could be built at a maintainer's request: for example, this file could be assembled anew when pushing from this repo's GitHub Actions workflow to https://github.com/amaranth-lang/amaranth-lang.github.io.

Yes please.

Done. I am deeply unhappy with the way it turned out, however; any suggestions as to how the code could be improved are appreciated.