Integrate OONI backend documentation into new docs website

[hellais]

This is about moving https://github.com/ooni/backend-documentation into https://github.com/ooni/docs

[hellais]

I would say this is done as part of https://github.com/ooni/docs/commit/a9e4f410b296a4331c81c1e5d3eada6fcdf6f6ac.

It also involved doing some restructuring of the pages.

Next steps involve:

• fixing some potentially broken links
• ensuring the text reads well even given the restructuring
• copy editing
• porting the charts over to mermaid