Exclude E2E tests for documentation-only PRs

[dtzar]

/kind feature

Describe the solution you'd like
When there are only documentation changes in a PR, ignore / do not run any of the E2E tests since this is a waste of time and money.

Some example PRs:
#3116
#3161

[CecileRobertMichon]

This is our current regex: https://github.com/kubernetes/test-infra/blob/master/config/jobs/kubernetes-sigs/cluster-api-provider-azure/cluster-api-provider-azure-presubmits-main.yaml#L60

Looks like the first one didn't run the e2e job, as expected. The second did, I believe because the .codespellignore file changed. We could exclude that too.

[dtzar]

@mboersma Did you have another job to provide as an example which unnecessarily ran e2e tests for just docs?

[mboersma]

#2805 only touched README.md but for some reason we ran the base e2e test and unit tests. (It was last November, so maybe we updated the regex since then?)

Other than that, I'm not finding any doc-only PRs that run unnecessary e2e tests. But #3230 only changes the Tiltfile, which also shouldn't cause any real tests to run, but did.

[CecileRobertMichon]

Another PR type that shouldn't be running e2e tests but does: OWNER file updates #3208

[mboersma]

#3268 only changes a markdown file, but ran all the tests.

[mboersma]

#3293 is another markdown-only change that spawned all the tests.

[CecileRobertMichon]

/assign

[CecileRobertMichon]

#3368 successfully didn't run e2e tests 🎉

[mboersma]

@CecileRobertMichon I also created #3366 and #3367 yesterday just to test that this is working. Looks good! Thank you for fixing this.