Pin mkdocs to < 1.6 to get docs building again

[sjpfenninger]

It seems the mkdocs 1.6 breaks the builds.

Summary of changes in this pull request

• Pin mkdocs to < 1.6 to get docs building again

Reviewer checklist

• [ ] Test(s) added to cover contribution
• [ ] Documentation updated
• [ ] Changelog updated
• [ ] Coverage maintained or improved

[codecov[bot]]

Codecov Report

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

Project coverage is 95.92%. Comparing base (872978d) to head (3f52d97). Report is 10 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #591 +/- ## ========================================== + Coverage 95.85% 95.92% +0.06% ========================================== Files 24 24 Lines 3619 3628 +9 Branches 788 723 -65 ========================================== + Hits 3469 3480 +11 + Misses 86 84 -2 Partials 64 64 ```