Make it explicit that the "main" docs page is actually devel doc
This is only symlinked since the impact of a plain renaming is not clear.
Motivation: Stop being confused at the /devel sphinx instance is actually on disk named "docs".
It was true as long as we only had one sphinx instance, we now have 3, so that no longer makes sense. And it's confusing at best.
Open for discussion, maybe we should just rename it?
Test Plan
make -C devel html
is happy
Migrated from D6470 (view on Phabricator)