docs(hosted-service): customer docs for the hosted Gallery service

[Deeds67]

Summary

docs.opennoodle.de covered self-hosting only — a customer who paid for the hosted
service and couldn't log in on their phone had nothing to read. Adds a new Hosted
Service sidebar section (position 1.5, right after Overview) with three pages:

• Signing up for hosted Gallery — what you get, EU data residency, checkout via
  Polar, the ~2-minute provisioning wait (sourced from the actual welcome-flow copy),
  and first login.
• Billing and your plan — how checkout/billing works, changing plans (upgrade =
  immediate/prorated, downgrade = end of period + storage gate), cancelling, and the
  two distinct erasure paths with their exact grace periods:
  • 60 days after a cancelled subscription actually lapses (not the day you click
    cancel) — DELETION_GRACE_DAYS = 60 / WARNING_DAY = 30 in
    deprovision.server.ts.
  • 7 days for the separate self-service "Delete account" option —
    DELETION_GRACE_DAYS = 7 in deletion.server.ts.
  • Plus the 14-day Widerruf withdrawal path (immediate, no grace period).
  • Plus how to export photos first (7-day download-link availability from
    export/constants.ts's EXPORT_TTL_DAYS).
• Logging in on the mobile app — the Noodle-branded app (App Store / Play Store
  links verified live), that sign-in goes through auth.noodlegallery.de, and the
  per-tenant server URL (https://<uid>.noodlegallery.de) to enter.

Links https://status.opennoodle.de wherever a customer would look for "is it down".

Every factual claim was verified against the platform repo (deletion.server.ts,
deprovision.server.ts, cancel.server.ts, switch.server.ts, account.tsx,
plans.tsx, teardown-export.server.ts, gallery-admin.server.ts, welcome/*) rather
than assumed — see the task report for the full verification trail. Nothing was invented
about UI copy I couldn't confirm (e.g. the mobile app's exact sign-in button wording is
described functionally, not quoted).

i18n: docusaurus.config.js has i18n.locales: ['en'] — this docs site is
English-only (unlike the marketing site's 7 locales), so no translation parity check is
affected by adding English-only pages.

Test plan

• pnpm install --filter documentation --frozen-lockfile + pnpm run build —
  succeeds, no onBrokenLinks failures (config has onBrokenLinks: 'throw').
• Confirmed all three pages exist in build/hosted-service/* and are linked from the
  rendered sidebar (href="/hosted-service/..." present in the built HTML).
• npx prettier --check docs/hosted-service/ — passes.
• CI — GitHub-hosted runners for this repo have historically queued for a long time
  when minutes run out; see PR checks for whether it actually ran.