serverguide: change all occurences of "optional" to "recommended"

[missytake]

Some headlines say "optional: " at the moment, even though we really recommend it.

Let's be a bit more paternalistic about this ;) if we wouldn't recommend it, it wouldn't be part of the guide.

[ralphtheninja]

Maybe move the reverse dns settings to the section https://delta.chat/en/serverguide#create-dns-entries ? Maybe also change the Name column values to the full host names? E.g. mail.example.org instead of just mail.