doc: Document ceph-mgr module configuration options

doc: Document ceph-mgr module configuration options

Add comprehensive documentation for defining configuration options in
ceph-mgr modules, including all supported properties and their usage.

Previously, the documentation did not explain how to define ceph-mgr
module configuration options, despite subtle differences from other Ceph
components. This change documents all supported Option properties, their
types, and provides clear examples to help module developers properly
configure their options.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
(cherry picked from commit 121192f4c95df0ee282cecc7237c8ca788e9cfba)