docs(connectors): document GMP over SSH for OpenVAS / Greenbone

[devGregA]

What

Documents the GMP over SSH transport in the OpenVAS / Greenbone connector reference, and splits the transport choice out from the rest of the setup.

Why

Greenbone carries GMP two ways, and the reference only described the older one:

• TLS on port 9390 was the default transport only through GOS 3.1.
• SSH is what Greenbone documents from GOS 4 onwards.

A current Greenbone commonly exposes no TLS listener at all — the Community Edition compose stack fronts gvmd with a unix socket and nothing on 9390 — so a reader following the existing instructions had no way to connect a modern instance.

What the section now says

• The transport choice up front, with which Greenbone version needs which.
• SSH prerequisites, covering both arrangements found in the field, since the connector detects which one a host uses rather than making the reader declare it: an appliance-style account whose forced command connects the session to gvmd (what gvm-tools expects, default account name gmp), or an ordinary account permitted to forward to gvmd's unix socket.
• That the GMP user is still required on SSH. This is the easiest thing to get wrong: SSH only carries the connection, and GMP still authenticates over it, so there are two sets of credentials rather than one.
• That the host key fingerprint field takes all of a server's fingerprints. A host usually offers several key types and there is no telling in advance which gets negotiated, so a single fingerprint read off the wrong key fails; ssh-keyscan <host> | ssh-keygen -lf - output pastes as-is.
• That Skip TLS Verification covers the TLS certificate only and does not accept SSH host keys — those have their own setting. Worth stating explicitly, because skipping the certificate check is routine against gvmd's self-signed cert, and it would be easy to assume that setting also governs host keys.
• The TLS instructions are kept, with the caveat that the Community Edition stack needs something in front of the socket (for example a socat TLS bridge) for that transport to be reachable.

Notes for review

Docs-only — one file, no code and no screenshots. The connector fields described here (Transport, the SSH * fields, gvmd Socket Path) ship in an upcoming connectors release, so you may want to land this alongside it rather than ahead of it.