#6994: Update Docs/hetzner

Updated the installation guide on a Hetzner VPS. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the Hetzner VPS installation guide, adding an optional `hcloud` CLI provisioning path, expanding the Docker Compose example (gateway + a CLI container), and adding a new “Configure Gateway” section with first-run commands and a validation/access flow (including SSH tunnel examples). The changes generally improve the operator experience, but there are a few documentation clarity issues that could cause confusion for first-time readers (e.g., undefined “bridge IP” note, and newly referenced `CLAUDE_*` environment variables not being introduced in the `.env` template). <h3>Confidence Score: 4/5</h3> - This PR is safe to merge; changes are documentation-only with a few minor clarity nits. - Review focused on the single changed doc file and the new/modified command snippets; no code paths or security-sensitive runtime logic were changed. Remaining issues are mainly about reader clarity and completeness of examples. - docs/platforms/hetzner.md (ensure the new notes and env var references are unambiguous) <!-- greptile_other_comments_section --> <!-- /greptile_comment -->