doc: set maxdepth to 2, so TOC isn't so long with new OSD troubleshooting.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added references from monitoring OSD to troubleshooting OSD.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Changed title to OSD and PG, indicating both subjects are covered.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added mention of Admin Socket interface and brief description.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added a lot of info to OSD troubleshooting.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>