doc/guile-ssh.texi (Top): Add `Concept Index'.

* doc/api-version.texi (Version): Update concepts.
* doc/api-sessions.texi (Sessions): Likewise.
* doc/api-messages.texi (Messages, Parsing of Requests): Likewise.
* doc/api-channels.texi (Channels): Likewise.
* doc/api-auth.texi (Auth): Likewise.
* doc/guile-ssh.texi (Top): Add `Concept Index'.
* doc/indices.texi (Concept Index): New node.