Add access from one docs dimension to another

We have now three dimensions following three personas for documentation:

• devel for contributors: https://docs.softwareheritage.org/devel/
• sys-adm for sysadmins: https://docs.softwareheritage.org/sysadm/
• and users for users: https://docs.softwareheritage.org/users/

In each dimension we need access to the other dimensions and/or access to a higher level page giving access to these dimensions.

• devel to sys-adm

• sys-adm to devel

• devel to user

• user to devel

• sys-adm to user

• user to sys-adm

---

Migrated from T3649 (view on Phabricator)