feat(documentation): Add indexes to operator categories in operator documentation sources

[PProfizi]

This PR updates the logic in ansys.dpf.core.documentation.generate_operators_doc.py to create .md index pages for each operator category and for the main operator-specifications.md file, enabling traversing the operator doc when generating it locally using DocFX.
It was previously impossible to access the operators or to list them by category as there is no navigation panel specific to operators when the doc is generated locally.

EDIT: since then, the local doc has been fixed to include the left navigation panel. However, this is still useful as this gives a landing page for each category, which makes the top navigation bar usable on the local doc.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.33%. Comparing base (1362534) to head (cf1c8fc).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2694      +/-   ##
==========================================
+ Coverage   84.28%   84.33%   +0.04%     
==========================================
  Files          92       92              
  Lines       10858    10890      +32     
==========================================
+ Hits         9152     9184      +32     
  Misses       1706     1706              

[github-actions]

Some tests with 'continue-on-error: true' have failed:

• PyDPF-Post API tests on ubuntu-latest

Created by continue-on-error-comment