CI: build docs on every PR but only deploy when merged to development

[SJaffa]

At the moment our documentation workflow only runs when a PR is merged to the development branch, because we don't want half-made features showing up in the official docs. But it would be good to test the documentation build without deploying it.

We could:

• have a new workflow that runs on open PRs and doesn't have the deploy step, or
• have a workflow with 2 separate jobs and the deploy step only runs on merge to development while the build step runs all the time.