Generate sphinx docs from help pages

[ianthomas23]

This adds sphinx documentation for the project, most of which is generated from the command and subcommand help by a python script create_markdown.py that runs at the start of the doc build process to generate the .md files that are used as input to sphinx. Fixes #72.

There is a separate help page for each subcommand, and there are links to the child subcommand help pages from the parent. Here is a screencast of the results of a local build:

git2cpp-docs.mp4

I have added a CI run to build the docs and upload an artifact so that we can manually check if the results are OK, and I've also added a Read the Docs config file with the intention of uploading there so that we have support for different build versions of the docs.

At this stage I am not looking for the output to be perfect, I just want to get the infrastructure in place so that we can improve the individual subcommand help and the appearance of the doc website.

[ianthomas23]

CI docs build works fine, the artifact is available at https://github.com/QuantStack/git2cpp/actions/runs/22581867209/artifacts/5723460686.

The ReadTheDocs setup will take longer to set up, presumably it needs some appropriate permission.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.83%. Comparing base (1ba9701) to head (47034e0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #110   +/-   ##
=======================================
  Coverage   86.83%   86.83%           
=======================================
  Files          60       60           
  Lines        2340     2340           
  Branches      276      276           
=======================================
  Hits         2032     2032           
  Misses        308      308           

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[ianthomas23]

Read the Docs build succeeded, it is at https://git2cpp--110.org.readthedocs.build/en/110/