Don't run code tests for PRs with only doc changes

Making doc improvements should be as easy as possible. Good documentation is important, so keeping the friction and time required to make doc changes as low as possible is important.

**Describe the solution you'd like**
Currently the test runs for PRs don't look for doc-only changes, and run the full suite regardless.
An improvement would be to have the test runner recognize doc-only changes.
At a minimum we want to do builds, but code tests are unnecessary (to the extent that such tests won't reference doc files).

This does require test runs to be able to determine what files have changed.
I don't know if this is possible.