Don't run a full build and test on changes to only `/docs/*`

[emily-potyraj]

• We'd like for the "build" action to be a required pass for PRs.
• We don't actually need to run the "build" action for docs.
• But if we ignore the /docs/** path, then docs-only PRs won't be able to merge. See [Bug] - Docs only PRs cannot be merged #209.

AB#1286

[rynowak]

This is done! Thanks @tcnghia