Add package documentation to buf.validate

[jrinehart-buf]

The package docs page for Protovalidate is the currently the third result if you do a Google search for "Protovalidate," but it doesn't provide any introductory text.

This PR uses a package-level comment to:

1. Adds the opening from its README.md file
2. Respects that this is package documentation ("This package provides the options, messages, and enums that power Protovalidate...")
3. Including a terse example
4. Provide links to both docs and all Protovalidate runtimes

Testing:

1. I've used a private repo to make sure it renders correctly:
   
2. I've used the playground to make sure the abbreviated proto compiles and validates correctly.

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Dec 5, 2025, 5:17 PM