Add context docs for nextjs

[paul-sachs]

Adds documentations about how to use contextValues with nextjs.

Context can be a little tricky so adding some additional docs to provide some direction.

[timostamm]

Thanks for the PR!

The goal for the package README files is to provide a concise minimum of information necessary to get started, and to link to the documentation and examples for details.

This isn't set in stone, and if there is a better approach, we can investigate it and make a consistent change.

However, until we do, the place to add this information would be https://connectrpc.com/docs/node/server-plugins#nextjs (where the contextValues callback is already documented, albeit not with a complete example), or to add an example to the examples-es repository.