Filter method docs

[RemDelaporteMathurin]

Proposed changes

Fixes #789 by adding an example in the post processing section for the filter method.

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

[codecov[bot]]

Codecov Report

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

Project coverage is 99.55%. Comparing base (84e7861) to head (c24b2eb).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #817 +/- ## ======================================= Coverage 99.55% 99.55% ======================================= Files 61 61 Lines 2707 2707 ======================================= Hits 2695 2695 Misses 12 12 ```

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