Fix api reference in readthedoc

[JeanMainguy]

The API reference generated by sphinx-apidoc was missing in our ReadTheDocs-hosted documentation. This problem occurred because ppanggolin and all Python dependencies weren't installed during the documentation build process. For more details, refer to this Stack Overflow.

To fix the problem:

• Added all Python dependencies in a specific category within the toml file.
• Modified the installation process in .readthedocs.yaml to install ppanggolin with the documentation and Python dependencies.
• Remove requirements.txt in docs folder as it was not used anymore
• Added the API Reference to the TOC tree of the Dev documentation section.

[jpjarnoux]

All good