Update docs on fenicsx

[RemDelaporteMathurin]

Proposed changes

Enough is enough! We have been neglecting the fenicsx branch documentation for too long!

• I took out most of the irrelevant stuff for FESTIM2 and added placeholder for now.
• We now test the documentation again!

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Copilot reviewed 61 out of 79 changed files in this pull request and generated no comments.

Files not reviewed (18)

• docs/source/api/index.rst: Language not supported
• docs/source/devguide/index.rst: Language not supported
• docs/source/environment.yaml: Language not supported
• docs/source/examples.rst: Language not supported
• docs/source/getting_started.rst: Language not supported
• docs/source/index.rst: Language not supported
• docs/source/installation.rst: Language not supported
• docs/source/userguide/beginners_guide.rst: Language not supported
• docs/source/userguide/boundary_conditions.rst: Language not supported
• docs/source/userguide/export_post_processing.rst: Language not supported
• docs/source/userguide/index.rst: Language not supported
• docs/source/userguide/materials.rst: Language not supported
• docs/source/userguide/mesh.rst: Language not supported
• docs/source/userguide/running_in_parallel.rst: Language not supported
• docs/source/userguide/settings.rst: Language not supported
• docs/source/userguide/simulations.rst: Language not supported
• docs/source/userguide/temperature.rst: Language not supported
• docs/source/userguide/traps.rst: Language not supported

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.49%. Comparing base (effcbac) to head (57683e2).
Report is 15 commits behind head on fenicsx.

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx     #931   +/-   ##
========================================
  Coverage    97.49%   97.49%           
========================================
  Files           44       44           
  Lines         2193     2193           
========================================
  Hits          2138     2138           
  Misses          55       55           

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