v2 onion services deprecation and documentation review

As v2 onion services are deprecating soon, let's review all the Onion Services page in TB-Manual and make sure the documentation points to v3 information. For example:"An onion address is a string of 16 (and in V3 format, 56) mostly random letters and numbers...", now we should say "a string of 56 mostly random letters and numbers...".

Let's use this ticket to review the page and track all the changes the need to be done.

https://tb-manual.torproject.org/onion-services/