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 NOAA-EMC#1791

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

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 need updates to the documentation. I have made corresponding changes to the documentation
• My changes generate no new warnings
• New and existing tests pass with my changes
• Any dependent changes have been merged and published

[github-actions]

Link to ReadTheDocs sample build for this PR can be found at:
https://global-workflow--26.org.readthedocs.build/en/26