CRAVEX-Docs: Rewrite AboutCode org Read the Docs

[tsteenbe]

Rewrite https://aboutcode.readthedocs.io to provide an better onboarding experience it should

• Provide an overview of key AboutCode projects so reader can quickly understand which project will likely address their needs
• Provide how-to or tutorials for use cases that spans across AboutCode projects following the to Diátaxis framework

[pombredanne]

This is implemented in this PR:

• CRAVEX-Docs: Improve docs for Getting Started purposes aboutcode#202