Update swh-docs pipeline after documentation merge

All documentation parts have been merged in a single Sphinx instance, resulting in a single build tree. The pipeline for swh-docs still tries to publish 3 different build trees.

added activity::Implementation label

Ah, yes, i thought this was already dealt with in the mr [1]. But this seems to only remove the upload part.

So yes, looks like there is some seek and simplifying some more that build. Looks like this is the part to drop [2]

[1] !154 (diffs)

[2] https://gitlab.softwareheritage.org/swh/infra/ci-cd/swh-jenkins-jobs/-/blob/master/jobs/templates/swh-docs-pipeline.groovy.j2#L67-92