UPD of developer's guide and docs for materials

[KulaginVladimir]

Proposed changes

Updates docs according to #884, #852, and #841

@RemDelaporteMathurin, let me know how the docs could be improved

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

Checklist

• Black formatted
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.55%. Comparing base (463b00b) to head (775ca3c).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #930   +/-   ##
=======================================
  Coverage   99.55%   99.55%           
=======================================
  Files          60       60           
  Lines        2725     2725           
=======================================
  Hits         2713     2713           
  Misses         12       12           

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[RemDelaporteMathurin]

Thanks a lot for this Vladimir! Grinding towards 1.4!!