Add filter to doc build action for PRs

[WalterKolczynski-NOAA]

Description

A path filter was applied to pushes, but none was applied to PRs, causing the GH action to build documentation to fire every time a PR is opened or updated.

Resolves #1791

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

This should work, but won't know for sure until it is merged. When using pull_request_target, the base branch is used for actions (see https://github.com/orgs/community/discussions/27084)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing tests pass with my changes

[github-actions]

Link to ReadTheDocs sample build for this PR can be found at:
https://global-workflow--1809.org.readthedocs.build/en/1809

[TerrenceMcGuinness-NOAA]

should work ... glad you did this, those constant messages with the link I set up where getting annoying, I should have thought of this myself when I started to see them.

[aerorahul]

Please get this in ASAP.