Email chapter needs a refactor

[joshtrichards]

The Email (outgoing SMTP/etc) chapter is pretty confusing. It could use some reorganizing and editing, to cut down on support requests, bug reports, etc.

https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/email_configuration.html

Additional topic ideas too (WIP):

• Why to use an ESP (e.g. Mailgun) rather than Gmail [note: low volume accounts are still generally free]
  • e.g. https://help.nextcloud.com/t/outlook-com-axioserror-request-failed-with-status-code-400/232443/4?u=jtr
• Oauth2 matters
• Conflicts between configuring in the Web UI versus via config file