Add roadmap to docs.s.o

As discussed on the devel mailing list, advertise the roadmap 2021 to the docs.s.o site currently held in a hedgedoc link [1].

> For the same reason I love your idea of putting it under docs.s.o, but I
> think it'd be fine under docs.s.o/devel/roadmap , linked from somewhere
> among the initial links at the index of docs.s.o/devel/. Any takers?

[1] https://hedgedoc.softwareheritage.org/s/VwWsTt5Wb

Migrated from T3208 (view on Phabricator)

Edited Jan 07, 2023 by Phabricator Migration user