docs: include the latest version in the switcher.

[miyukoc]

This addresses an issue where, if the user navigates to the documentation site for the latest version (/docs/amaranth/latest/), an extraneous entry will be prepended to the version list that matches the currently open version but does not show up in docs for other versions (e.g. /docs/amaranth/v1.2.3/).

This patch adds an entry for the latest version at build time and removes the behavior where an extraneous entry would be prepended to the version list if no entry that matches the currently open version is present.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.28%. Comparing base (e11cb25) to head (bc07bfd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1644   +/-   ##
=======================================
  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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