Organize documentation

[fv3rdugo]

Documentation has grown and should be rearranged to make it more accessible. Now as shown on the left at https://ravada.readthedocs.io the entries are aligned to infinity.

The main documentation for the site is divided into three main sections:

• Administrator Documentation
• Feature Documentation
• Guest VM section

Describe the solution you'd like Edit and reorganize entries in index.rst

Describe alternatives you've considered I'm inspired in https://docs.godotengine.org/en/stable/