Add pages for each protocol / protocol family

[fingolfin]

We currently have pages for each SSH implementation. We might want to also provide pages for each protocol (such as "aes128-ctr"), or protocol family (such as "aes*-ctr", i.e. "aes128-ctr,aes192-ctr,aes256-ctr").

On these pages, we could

• list clients supporting the protocol
• link to the relevants RFCs / specs
• whether it is required / recommended / optional / not listed in the RFC
• add some notes, e.g. with pointers to known weaknesses, whether the protocol is known to be unsafe, etc.

Some starting places:

• https://news.ycombinator.com/item?id=8843994
• https://stribika.github.io/2015/01/04/secure-secure-shell.html
• For aes128-gcm@openssh.com, aes256-gcm@openssh.com: point to http://www.openssh.com/txt/release-6.2 and mention
  "[...] uses an identical packet format to the
  AES-GCM mode specified in RFC 5647, but uses simpler and different
  selection rules during key exchange."