Documentation guide

[RemDelaporteMathurin]

Proposed changes

Fixes #778 by adding a guide to building the documentation in the docs.

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
• [x] Documentation Update (if none of the other choices apply)
• [ ] New tests

[RemDelaporteMathurin]

@KulaginVladimir do you think there's anything I missed here?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.53%. Comparing base (36da67b) to head (94fccf1).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #779 +/- ## ======================================= Coverage 99.53% 99.53% ======================================= Files 59 59 Lines 2577 2577 ======================================= Hits 2565 2565 Misses 12 12 ```

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