docs: Explicit the folder to deploy docs to

As a requisite for this to work, the global option "Remote directory" [1] got modified
from /srv/softwareheritage/docs/devel to /srv/softwareheritage/docs/webroot. So we can
push the users, devel or future new docs to their respective folders.

[1] https://jenkins.softwareheritage.org/configure

Related to T3388