📝 Switch to `furo` theme for documentation

[burgholzer]

Description

This PR changes the default theme for the documentation to furo. The main reason being that the sphinx-rtd-theme isn't well maintained and has led to multiple issues. As a nice side effect, fury makes light and dark mode a breeze and makes code look nice in both cases without any manual css edits.

This PR also updates the RtD CI configuration to use newer Ubuntu runners and Python 3.11.

Checklist:

• [x] The pull request only contains commits that are related to it.
• [x] I have added appropriate tests and documentation.
• [x] I have made sure that all CI jobs on GitHub pass.
• [x] The pull request introduces no new warnings and follows the project's style guidelines.

[codecov[bot]]

Codecov Report

Merging #286 (45181de) into main (681649a) will decrease coverage by 0.2%. The diff coverage is n/a.

@@           Coverage Diff           @@
##            main    #286     +/-   ##
=======================================
- Coverage   96.2%   96.0%   -0.2%     
=======================================
  Files         34      34             
  Lines       1712    1713      +1     
  Branches     208     208             
=======================================
- Hits        1647    1645      -2     
- Misses        65      68      +3     

Flag	Coverage Δ
cpp	95.6% <ø> (-0.3%)	:arrow_down:
python	97.3% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mqt/qcec/verify.py	100.0% <ø> (ø)
mqt/qcec/verify_compilation_flow.py	100.0% <ø> (ø)

... and 2 files with indirect coverage changes