Test documentation navbar sorting script

[philipandag]

The script can be used to sort specific sections of a text file, like the navbars in documentation.

The script usage:
<script> [file-to-sort] [start-mark] [end-mark]
file-to-sort is mkdocs.yml by default. The start & end marks are #pre-commit-sort-start and #pre-commit-sort-end by default, which can be seen in mkdocs.yml file.

If we would like to use it then it should definitely be added as a pre-commit hook, not as a loose script. I had problems with creating a working pre-commit hook , so I'll push this script as is for now.

Merge after #955, if at all

*EDIT
Turns out the script can be easily run if it's present in the repo locally. I've changed the .pre-commit-config to run it only on the "mkdocs.yml" file and it works fine. It would be better to move it to a different repository, but I haven't figured out how to do that.