Re-use docs builds

[trexfeathers]

On SciTools/iris, we currently have three workflow jobs that begin with building documentation, currently managed via Nox sessions:

• doctest
• gallery
• linkcheck

These are all invoked within the same GHA jobs matrix.

Docs building is one of the most resource-heavy steps in Iris CI, and these jobs are often the longest running. There is an opportunity here to have all three jobs run using a single instance of built docs, saving GHA resources:

• Gives more headroom with the GHA concurrency limit.
• Is the right thing to do as a Force for Good in the GitHub/open-source community.