Documentation for importing proto files

Importing and sharing proto files is a common tripping point. Grpc.Tools doesn't really have docs.

Document this to help people and provide a link that we can point people towards.

Start by researching how Grpc.Tools imports and shares proto files, then inspect the repository's existing documentation structure for the appropriate location. Done means adding a practical guide for this workflow and a link that maintainers can share with users.

Written by the indexing model from the issue text.