fenicsx installation instructions

[RemDelaporteMathurin]

Proposed changes

This adds installation instructions for the fenicsx branch.

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.55%. Comparing base (a9d8980) to head (c99a135).
Report is 14 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #917   +/-   ##
=======================================
  Coverage   99.55%   99.55%           
=======================================
  Files          60       60           
  Lines        2717     2717           
=======================================
  Hits         2705     2705           
  Misses         12       12           

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