Autopopulate API contents if none provided

[cliffckerr]

Summary

For small Python packages, you typically want to document the entire API, and don't need to do any customization in terms of ordering. But currently, quartodoc requires the API contents to be explicitly listed in _quarto.yml.

This PR makes it so if no contents are provided to quartodoc, it will behave as if every source file was listed.

This behavior was copied from mkdocstrings-python, since this was the only feature I could find that mkdocstrings had that quartodoc seemed to lack :)

Details

• Auto-populate a section's contents from the configured package when none is provided, so users can omit explicit content lists for full-package documentation.
• Walks the package's submodule search locations for *.py files (excluding __init__.py and __pycache__) and generates an Auto entry per module.
• Adds a validator on Page.contents to reject empty page contents, and re-exports validator from the pydantic v1/v2 compatibility shim.
• Documents the new behavior in docs/get-started/overview.qmd.

Test plan

• Build a site with a quartodoc config whose section omits contents and confirm modules are auto-discovered.
• Build a site with explicit contents and confirm behavior is unchanged.
• Confirm a warning is logged when a section has no contents and no package is configured.
• Confirm a Page with empty contents raises a validation error.
• Verify the new docs section renders correctly in docs/get-started/overview.qmd.

(NB: feature implemented with the help of Claude)