📚 DOCS: Convert autodoc elements to Markdown

[mbercx]

Currently the automatically generated parts of the documentation are still in RestructuredText:

• [ ] The autodoc generation of the API. There is also https://sphinx-autodoc2.readthedocs.io/en/latest/index.html. I remember there was a reason I didn't use it, but I don't remember why. ^^
• [ ] Automated generation of CLI docs.
• [ ] aiida.sphinxext items

It would be good to switch to an entirely Markdown formatted documentation for consistency. I think it would also be possible to have docstrings in Markdown, which again would make it much easier to write docstrings that

1. Render nicely in the API docs.

2. Can be very clear when hovering over a method in IDE's that support it, like VSCode:

   

I've found [2] to be very valuable when doing development. However RestructuredText-like elements like :param X: and :returns: aren't rendered:

Maybe there is an extension for this, but if we can just do everything in Markdown, I think our lives would become easier (no more thinking if we need single or double backticks somewhere 😉).