Draft: Document running Relay through OAK on a Linux package

⚠️ Experiment, not to be merged. Part of the Relay-on-Theseus PoC (Refs #1082). The findings are summarized in Relay on Theseus: what's missing.

Adds doc/oak.md, the operator guide for running Relay in an OAK cluster next to a Linux package install: what changes, prerequisites, service exposure, the gitlab.rb block, what reconfigure writes, creating the Secrets and installing the chart, verification, security notes and known gaps. Describes the omnibus-gitlab branch of this experiment; nothing here is in a released package.

MRs of this work:

• Draft: Add a Relay container image with the GLAZ engine linked in (!4313 (merged))
• Draft: Add a Fairway-generated Helm chart for Relay (!4315 (closed))
• Draft: Add a Relay fragment with AutoFlow on CNPG (gitlab-org/gitlab-caproni!108 (closed))
• Draft: Add identified PostgreSQL databases, roles, and TLS to gitlab.infrastructure.v1 (gitlab-org/protos!38 (closed))
• Draft: Mount several PostgreSQL databases, roles, and TLS Secrets from the infrastructure contract (gitlab-com/gl-infra/platform/runway/fairway!217)
• Draft: v2/postgres: select databases and roles from the infrastructure contract (gitlab-org/labkit!666 (closed))
• Draft: Read autocore's databases from the Theseus infrastructure contract (!4317)
• Draft: Declare several CloudSQL databases on a deployment and render them through one proxy sidecar (gitlab-com/gl-infra/platform/runway/runwayctl!1958)
• Draft: Switch the Relay chart to the infrastructure contract (!4318)
• Draft: OAK: add GitLab Relay as a component (gitlab-org/omnibus-gitlab!9742 (closed))
• Draft: Document running Relay through OAK on a Linux package (!4319)