docs: add docker-compose.override.example.yml and Rucio integration guide

[veprbl]

The RucioStager plugin (PR #31) has a documented opt-in in its own docstring, but the wiring the plugin needs to actually reach a running Rucio is spread across four places:

• A rucio.cfg + CA cert + OpenSSL hash symlink mounted into the three services that talk to Rucio (panda-jedi, panda-server, harvester).
• A shared Docker network so the hostname 'rucio' resolves inside the panda-compose containers.
• RSE-storage and per-worker-output bind mounts on the harvester service so the stager can read output files and write to the MOCK-POSIX RSE.
• A rucio-clients package install in the harvester's pip line so the plugin can import UploadClient.

None of that is discoverable from the panda-compose repo today; anyone opting into RucioStager has to rediscover it from scratch or copy an undocumented file from someone else's tree.

Add:

• docker-compose.override.example.yml — a fully documented example override that wires all three services to an external ruciodevnetwork and mounts the Rucio configs. Compose picks up docker-compose.override.yml automatically, so users copy the example, edit if needed, and rerun 'docker compose up -d'.
• docs/rucio-integration.md — walks through the five opt-in steps (rucio.cfg, override, panda_queues.cfg, rucio-clients pip, restart) and points at the plugin's own docstring for the tradeoffs.
• docs/_sidebar.md — link the new page under 'Integration'.
• .gitignore — ignore docker-compose.override.yml so users' local edits don't accidentally get committed.

No default behavior change: the override file is only picked up if the user explicitly copies the example.

[wdconinc]

Is this already something that can be exercised in CI as well? Or don't we have a docker-compose-able rucio instance yet?

[veprbl]

No, not yet, but I'm planning to build up to that point.