Documentation builds not triggered when README.md, CONTRIBUTING.md, LICENCE.txt altered

[mfisher87]

These files are symlinked into the docs directory, and our workflow is triggered by changes in the docs directory. GitHub Actions does not recognize a change to README.md as a change to the symlink docs/index.md, so if the README is changed, docs don't build. I tested this on a fork.

mfisher87#4

[jrbourbeau]

Circling back here. I think the recent readthedocs work closes this issue as now we get a documentation build + preview on all PRs. I'm going to close this issue as fixed, but feel free to re-open is that's not the case @MattF-NSIDC

[mfisher87]

I think we need to clean up and simplify some GHA YAML:

• Ignore Pull Request event - RTD does this
• Update paths list for push events - we want GH Pages docs to match RTD when README.md is changed, for example.
• Simplify this conditional, should just be checking if branch == main|documentation - the rest is vestigial IIRC

[mfisher87]

I'm opening a quick PR so we don't have to open anything else to track that :)

EDIT: #324