Clean up CiviMail docs

Created by: seancolsen

Relevant pages:

• https://docs.civicrm.org/sysadmin/en/latest/setup/civimail/
• https://docs.civicrm.org/sysadmin/en/latest/setup/civimail/outbound/
• https://docs.civicrm.org/sysadmin/en/latest/setup/civimail/inbound/

I brought stuff into this chapter from (a) the User Guide and (b) a bunch of different pages on the wiki. Now it's kind of a mess and has some duplicate content. Needs merging, cleaning, and some re-organizing.