Post link in PRs when RTD docs have been built

[WalterKolczynski-NOAA]

Description
We have turned on documentation building for PRs at ReadTheDocs. The link is of the form https://global-workflow--{pr_number}.org.readthedocs.build/en/{pr_number}/. But the only way to get there is either knowing the link or getting there through the RTD project dashboard.

Requirements
A comment on the PR with a link to the RTD documentation built from the PR.

Acceptance Criteria (Definition of Done)
Meet requirements.

(Optional): Suggest A Solution
Not sure whether the best solution is to add it to the existing GH action, or use some sort of webhook.

Also, with documentation being built for PRs on RTD, I don't know what parts of the existing GH action are still even needed. At the very least, we likely do not need to upload the artifacts anymore. Will need to evaluate how much the RTD build relies on our GH action.