docs: document ownCloud Web merge-in and AGPL→Apache-2.0 relicensing (#12491)

Automated documentation follow-up for a merged, user-visible change.

### Source
PR #12491 — *[full-ci] chore: [OCISDEV-5] merge and relicense ownCloud Web into oCIS* (merged to `master` 2026-07-07). Fixes #11211.

### What changed
- The `owncloud/web` frontend is now imported in-tree at `web/` inside this repository, byte-exact against the final standalone `v12.5.0` release. `owncloud/web` remains archived as the historical record up to that point — it is no longer where web/frontend contributions or issues should go.
- The `web/` tree is **relicensed from AGPL-3.0 to Apache-2.0**, matching the rest of oCIS. Root `NOTICE` documents the relicensing provenance; `REUSE.toml`/`LICENSES/` were added for REUSE 3.3 compliance.
- **Build/deployment change:** `services/web/Makefile` now builds web assets from the in-tree source instead of downloading a release tarball from `owncloud/web`. Anyone with a custom/vendored build pipeline that fetched a `owncloud/web` release tarball needs to switch to building from the in-tree `web/` directory.
- CI: e2e tests no longer clone `owncloud/web`; new workflows (`web-tests.yml`, `check-licenses.yml`, `commit-message.yml`, `release-npm-packages.yml`) replace web's old standalone CI. `release-npm-packages.yml` is not yet functional (needs npm trusted-publishing/`NPM_TOKEN`).

### Documentation to update
- [ ] **Licensing docs / README** — reflect that `web/` is now Apache-2.0, not AGPL-3.0; note the relicensing provenance (CLA-covered contributions) for downstream/legal audiences.
- [ ] **Contribution guide** — web/frontend contributions now go through `owncloud/ocis` (this repo), not `owncloud/web`. Update any contributor-facing links/instructions that still point at the standalone repo.
- [ ] **Build/packaging docs** (`docs/services/web/releasing.md`, deployment examples) — confirm downstream packagers relying on the old "download a `owncloud/web` release tarball" build step are pointed at the new in-tree build process.
- [ ] **Repo README / architecture overview** — note that oCIS is now a single-repo product (backend + web frontend), not two coordinated repos.

### Notes
- This PR's own description already covers most of the above at a technical level; this issue tracks turning that into end-user/admin/contributor-facing docs, which isn't covered by a changelog fragment alone.
- No runtime config options, CLI flags, or API endpoints changed — this is a repo-structure, build-process, and licensing change only.

_Opened automatically by the oCIS triage routine (PR-merged trigger). Documentation PRs are not auto-merged._

🤖 Generated by Claude Code