docs: expand documentation for sample_metadata()

[Tanisha127]

This PR expands the documentation for sample_metadata() to provide:

• A clearer summary of how metadata is assembled
• Detailed parameter descriptions
• An expanded description of the returned DataFrame
• Explanation of merged metadata sources (general metadata, QC metadata, surveillance flags, AIM, cohorts)

Fixes #553.

[Tanisha127]

Hi @jonbrenas and @ahernank,

I’ve opened PR #933 to expand the documentation for sample_metadata().
I’d appreciate your feedback when you have time.
Also, the workflows are awaiting maintainer approval for CI to run.

Thank you!