docs: document sshd configuration and client keepalives

[giuliocalzo]

Documents the generated sshd_config and how to extend it, instead of changing any chart defaults. Supersedes #254, which proposed baking keepalives into values.yaml; the review preference upstream was to keep all extra* fields empty and document the configuration, which is what this does.

Adds docs/usage/ssh-access.md, covering:

• How sshd_config is assembled. The operator generates the whole file and mounts it over /etc/ssh/sshd_config, so the image's own defaults do not apply — worth stating, since it is invisible from the chart.
• Client keepalives. Nothing sets ClientAliveInterval, so it stays at OpenSSH's 0 and sshd never probes its clients. Long-lived sessions are then dropped by the idle timeout of whatever sits in front of them (350s AWS NLB, 600s GCP, 4 min Azure; TCPKeepAlive does not help, as Linux waits two hours). The reverse case is a vanished client leaving a session that, on a worker, still holds its adopted job. Documented with the reasoning for choosing an interval rather than just a snippet to paste.
• Settings the operator already defines. sshd_config takes the first value per keyword and extraSshdConfig is appended last, so keywords the operator already emits cannot be overridden there — the added line is silently ignored. The page lists them for login and worker pods and points at an Include drop-in for changing them. This is also why ClientAlive* is legitimately an extraSshdConfig setting: it does not collide with anything.

Docs only, no chart or code change. Formatted with the repo's mdformat --wrap=80 pre-commit hook; the page is picked up automatically by the usage/* toctree glob.

[SkylerMalinowski]

Merged: 334c26f