Add package documentation for Protobuf packages

[bufdev]

So the BSR can pick it up. Any file in each package can have a comment on the actual package declaration, and the first comment by filename in alphabetical order will be picked up.

[stefanvanburen]

Looks like this was tackled in #448, package docs visible at https://buf.build/bufbuild/protovalidate/docs/main:buf.validate. Going to call this done.