* Add GitHub Actions documentation-deployment pipeline

This brings a documentation-deployment pipeline into the Qiskit/Terra
repository, allowing it to fully deploy its documentation to
`qiskit.org`, a task previously only the metapackage could perform.
This does not fully unify the documentation with files from the
metapackage, it just adds a pipeline to do the final deployment.

This includes a revitalised translatable-strings pipeline, which was
previously broken on the metapackage for the last month or two. It also
previously included a fair amount of legacy weight that was no longer
relevant.

* Add missing secret insertions

* Improve logic for deployments

This changes the logic for the deployments so that pushes to 'stable/*'
no longer trigger any deployment to qiskit.org.  Instead, tag events
trigger a deployment to the relevant stable branch, and a tag event of
the _latest_ tag triggers a deployment to the documentation root.

The translatables logic is modified to push only the latest full-release
tag.

(cherry picked from commit 80e95d1)

# Conflicts:
#	docs/conf.py

For any tag we publish from the `stable/0.25` branch the publish path
should be both `stable/0.44` (which matches the qiskit package version) 
and the root as it's the current release. When we stop publishing from
the stable/0.25 branch after the release of 0.45.0rc1 the existing logic 
will work and we'll no longer have any 0.25.x releases. But for the
0.25.x release series only the version numbers are not unified so we
need to manually ensure we're publishing to the right paths.