doc: update mgr module command documentation for per-module registries

doc: update mgr module command documentation for per-module registries

Update documentation to reflect the new per-module command registry
pattern introduced in PR #66467. The old global CLICommand decorators
have been replaced with module-specific registries.

Changes:
- doc/mgr/modules.rst: Rewrite CLICommand section with setup guide,
  update all examples to use AntigravityCLICommand pattern
- src/pybind/mgr/object_format.py: Add note explaining per-module
  registries and update all decorator examples
- doc/dev/developer_guide/dash-devel.rst: Update dashboard plugin
  examples to use DBCLICommand

All examples now correctly show:
- Creating registry with CLICommandBase.make_registry_subtype()
- Using module-specific decorator names (e.g., @StatusCLICommand.Read)
- Setting CLICommand class attribute for framework registration

Signed-off-by: Kefu Chai <k.chai@proxmox.com>