doc: Replaced "plugin" with "module" in the Mgr documentation

doc: Replaced "plugin" with "module" in the Mgr documentation

The documentation currently refers to Ceph Manager Modules as
"plugins" in many places, while the command line interface uses
"module" to enable/disable modules. Replaced all occurences
of "plugin" with "module" in the docs, to avoid confusion and to
be in alignment with the CLI. Also fixed the capitalizations of some
module chapters.

Fixes: https://tracker.ceph.com/issues/38481
Signed-off-by: Lenz Grimmer <lgrimmer@suse.com>