metadataValidator: workflow integration

[krlong68]

We want to integrate the metadata validation functions (metadataValidator.R, validate_metadata.qmd) into a GH actions workflow to automatically run whenever the metadata schema is updated or a new study is curated.

[lwaldron]

Can close once GHA is running successfully (https://github.com/waldronlab/curatedMetagenomicDataCuration/actions/runs/20847328041/job/59893899311) and updates the pkgdown site (https://waldronlab.io/curatedMetagenomicDataCuration/)

[shbrief]

Using .github/workflows/validate-metadata.yml for validation. The GitHub page is ready: https://waldronlab.io/curatedMetagenomicDataCuration/

[seandavi]

The resulting site is BEAUTIFUL!