add automated docs building and publishing

[briantist]

SUMMARY

As discussed: ansible/community#644 (comment)

I will wait until we're sure it's working before adding a new PR deleting the RST files in docs/ and the related workflow.
Follow-up PR is up: #392

ISSUE TYPE

• Docs Pull Request

[briantist]

Following discussion in WWG, I've modified the PR workflow to remove the publishing step, which is the step that runs with contents: write permission.

What's left is the build job (read-only, only part that touches PR code), and the separate job for posting a comment on the PR, and doesn't have contents access at all.

This will let us still:

• verify that PRs are not causing docs problems
• post a helpful comment showing (in diff format) what's changed, and where to download the rendered docs for local review

---

The push/schedule workflow remains the same, with build + publish

[nitzmahone]

Looks as safe as it can be for the push cases- thanks!