Document invoke_service extension hook

[samuel-williams-shopify]

Document Async::GRPC::Dispatcher#invoke_service as the supported protected extension point for wrapping service execution.

The documentation covers:

• middleware and instrumentation use cases
• the requirement to call super
• stream cleanup, trailer, and status responsibilities
• the hook boundary for routing/setup errors and deadline expiry
• an example override

Validation:

• bundle exec rubocop lib/async/grpc/dispatcher.rb
• bundle exec bake decode:index:documentation lib/async/grpc/dispatcher.rb
• bundle exec bake test