Create main page for docs.softwareheritage.org

By having a main page in the docs repo from which a dev doc can be access, we can add other sections to the documentation that is not developers oriented.

The proposed structure during the March docs sprint is the following:

Proposed structure of main page

(https://docs.softwareheritage.org/)

• devel (contributors) (goal: code will probably land in SWH)
  • tutorials
  • howto
  • reference
• devel (users) -> probably merge with previous item (very large intersection) (goal: their own codebase, but either use SWH services or SWH librariesm)
• use (non-technical)
  • tutorials
  • howto
  • explanation
• sysadm
• FAQ

This structure should be reviewed with the exact structure of each child-page.

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Migrated from T3155 (view on Phabricator)