Recommend authentication via ssh key with ed25519 algorithm (#231)

[hagenest]

this should fix #231 (and arguably #287) or would you prefer a longer explanation? I felt a long paragraph would be overkill in comparision to the rest of the short documentation.

I haven't written into the changelog, as this is doesn't touch the code, is this the right way?

[link2xt]

Would be also nice to describe that you need to have key without password or use ssh-agent. Otherwise ssh tries to ask for password when you run pyinfra.