docs: Sphinx 9 support and MyST-parser fixes

[flying-sheep]

• Closes #
• Tests included or not required because:

• Release notes not necessary because: no(?) user-visible changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.16%. Comparing base (d5901ab) to head (4fe4d24).
⚠️ Report is 52 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3948   +/-   ##
=======================================
  Coverage   78.16%   78.16%           
=======================================
  Files         117      117           
  Lines       12586    12586           
=======================================
  Hits         9838     9838           
  Misses       2748     2748           

Flag	Coverage Δ
hatch-test.low-vers	77.49% <ø> (ø)
hatch-test.pre	77.12% <ø> (ø)

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