docs(hub): add CLI, config, and multi-machine guide pages

Per PR #139 review, every code change ships with its docs. This adds:

- reference/cli.md: `openusage hub` and `openusage hub-view` sections
  with flags (incl. --allow-public), endpoint table, auth posture, and
  the unsafe-default guard error message verbatim so operators can grep
  for it.
- reference/configuration.md: new `export` and `hub` sections, with a
  warning that auth_token is env-var-only and never persisted to
  settings.json (mirrors the `api_key_env` story).
- reference/env-vars.md: OPENUSAGE_HUB_TOKEN entry.
- guides/multi-machine.md: end-to-end walkthrough — architecture
  diagram, three auth postures, hub startup (process and Docker), per-
  worker exporter setup, viewing options, and operational notes about
  stale eviction and bind safety.
- sidebars.ts: registers the new guide under Guides.

Verified with `cd docs/site && DOCS_PREVIEW=1 npm run build` — [SUCCESS]
with no warnings or broken-link errors.

Refs: https://github.com/janekbaraniewski/openusage/pull/139#pullrequestreview-4306373001