Add documentation for gRPC-based cluster registration

This commit adds comprehensive documentation for the gRPC-based
registration feature introduced in OCM v0.15.0, addressing issue #521.

Key additions:
- New guide: register-cluster-via-grpc.md covering:
  - Architecture and benefits of gRPC registration
  - Complete ClusterManager configuration with registrationDrivers
  - Instructions for exposing gRPC server via Ingress/Route
  - Klusterlet configuration with gRPC authType
  - Troubleshooting guide and migration instructions
  - Important note about add-on limitations with gRPC

- Updated start-the-control-plane.md to reference the new gRPC
  registration option as an alternative to standard registration

The documentation provides accurate API configurations based on the
open-cluster-management-io/api repository and includes practical
examples for both Kubernetes and OpenShift environments.

Fixes #521

Signed-off-by: Jian Qiu <[email protected]>

Author: qiujian16