Add pack-metadata.yaml

[dcmcand]

Adds pack-metadata.yaml at the repo root so this pack renders on the Nebari software pack dashboard, which fetches this file hourly.

Heads up: these field values are a best guess and likely need fixing before merge. I filled them in without authoritative input, so please check them. The ones most likely to be wrong:

• owner: guessed as pmeier. Set this to whoever is actually accountable for the pack.
• product_owner: left null. It is only required at level: ga, but set it now if this pack has one.
• level: set to experimental as a conservative default. Bump it if the pack is further along (see the release-readiness checklist).
• docs link: omitted. Add a links.docs entry if there is documentation beyond the README.

description is intentionally omitted: the dashboard sources the Description column from the GitHub repo description, not this file. Other optional fields are left commented out.

Depends on nebari-dev/software-pack-dashboard#1 (makes description optional in the schema). Merge that first to avoid a transient metadata-invalid flag on the dashboard.