Cleanup documentation for 8.4.0

[kdeldycke]

A WIP PR that I plan to use to fix all the small stuff we missed during the 8.4.0 release cycle and merging frenzy.

For the moment, this:

• Adds the missing changelog entry for Extract _guess_type from convert_type and add overloads #3372 as requested in Extract _guess_type from convert_type and add overloads #3372 (comment)
• Cleanup Fix: Ensure fish completion handles multiline help strings correctly #3126: link to PR in changelog, move implementation details to Python code, fix changed version and align .gitignore syntax
• Add missing changelog entry for Localisation of more strings #2902

Before considering for a merge, do a full pass review on the big diff: 8.3.3...main

[Rowlando13]

Let me know when this is ready. I have been pretty careful about missing changelog entries and this catches the ones I am tracking, so I don't think there is a need to look at the big diff.

[kdeldycke]

Let me know when this is ready. I have been pretty careful about missing changelog entries and this catches the ones I am tracking, so I don't think there is a need to look at the big diff.

Given that we're not going to add more feature for 8.4.0 as per your comment at #3404 (comment), this PR is ready to be merged! :)