Clean up commands doc after mcp experimental changes

[hasheddan]

Description of your changes

Cleans up documentation for control planes commands after the changes to the control planes API. Drops the attach and token commands, and adds documentation for the --mcp-experimental flag.

Signed-off-by: hasheddan georgedanielmangum@gmail.com

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

🤖 (up) up ctp -h
Usage: up controlplane <command>

Interact with control planes.

Flags:
  -h, --help                         Show context-sensitive help.
  -v, --version                      Print version and exit.

      --mcp-experimental             Use experimental managed control planes API ($UP_MCP_EXPERIMENTAL).
      --domain=https://upbound.io    Root Upbound domain ($UP_DOMAIN).
      --profile=STRING               Profile used to execute command ($UP_PROFILE).
  -a, --account=STRING               Account used to execute command ($UP_ACCOUNT).

controlplane
  controlplane create <name>
    Create a hosted control plane.

  controlplane delete <id>
    Delete a control plane.

  controlplane list
    List control planes for the account.

  controlplane kubeconfig get --token=STRING <control-plane-ID>
    Get a kubeconfig for a control plane.