Add documentation for gRPC-based cluster registration (#526)

* 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 <jqiu@redhat.com>

* docs: update gRPC registration guide to focus on LoadBalancer

- Remove OpenShift Route-specific content
- Remove Ingress option (not tested)
- Focus on LoadBalancer service approach with cloud provider examples
- Clarify that custom CA bundles are not supported
- Add AWS EKS annotation example for Network Load Balancer

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Signed-off-by: Jian Qiu <jqiu@redhat.com>

* docs: address PR review feedback on gRPC registration guide

Address all feedback from PR review #526:
- Update version requirement from v1.1.0 to v1.2.0
- Add clusteradm to prerequisites
- Add clusteradm init option as recommended approach
- Change grpc type from hostname to loadBalancer
- Document automatic LoadBalancer service creation
- Remove manual LoadBalancer service creation step
- Add clusteradm join with gRPC flags as recommended option
- Fix kubectl commands to include resource name (kubectl edit klusterlet klusterlet)
- Add -k flag to curl commands for testing
- Change kubectl get to kubectl describe for service verification
- Update all service references from cluster-manager-grpc-server-lb to cluster-manager-grpc-server
- Update troubleshooting section to reflect automatic LoadBalancer creation
- Clarify when manual configuration steps can be skipped

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Signed-off-by: Jian Qiu <jqiu@redhat.com>

* docs: fix kubectl command spacing inconsistencies

Remove extra spaces in kubectl commands for better readability.

Signed-off-by: Jian Qiu <jqiu@redhat.com>

* docs: add klusterlet resource name to kubectl get command

Fix kubectl get klusterlet command to include resource name for consistency.

Signed-off-by: Jian Qiu <jqiu@redhat.com>

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Signed-off-by: Jian Qiu <jqiu@redhat.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: qiujian16