Except doc files from CI runs

[thenorili]

Running CI over changing the README is a waste! I just tested this in a personal repo of mine and was delighted that it works. I can't demonstrate that it works in this pull request 'on pull request' considers all files changed in the PR and this change necessarily changes the ci.yml, but I can show where it's worked elsewhere.

Note this isn't compatible with the "Require status checks to pass before merging" branch protection setting. I wasn't sure if we used that, but it looks like we do have a number of required checks that this would cause trouble for -- the CI wouldn't run automatically, so docs PRs would be left stuck.

[thenorili]

https://github.com/github/docs/issues/8926 https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/collaborating-on-repositories-with-code-quality-features/troubleshooting-required-status-checks#handling-skipped-but-required-checks

Looks like the recommendation is to separate the required checks into a separate workflow.