chore(sync): retire the upstream mirror and author content in-repo

[DeepDiver1975]

Executes the first bullet of Phase 5 in #44:

Disable/remove sync-upstream.yml; mark sync/manifest.yml + sync/ historical (dedicated PR).

What changes

• Deleted .github/workflows/sync-upstream.yml. Nothing mirrors the legacy owncloud/docs-* repos into this repo any more.
• sync/ retired, not removed. sync/manifest.yml keeps the branch→folder mapping as provenance: it records which upstream repo and branch each version folder was last imported from, which is exactly what you need to diff a folder against its legacy origin.
• sync/sync-repo.sh gains a retirement guard. The mirror is destructive — rm -rf + cp -a per sync_paths entry — so a stray invocation would silently discard every in-repo edit made since the cutover. It now refuses unless SYNC_UPSTREAM_I_KNOW_THIS_IS_RETIRED=1 is set.
• sync/patches/README.md notes the two patches (docs-webui.sh, docs-main.sh) are already materialised in the tracked content, so the corrected xrefs are permanent and need no re-application. Verified in content/webui/modules/ROOT/pages/index.adoc, content/main/modules/ROOT/partials/nav.adoc and content/main/modules/ROOT/pages/server_releases.adoc.
• README.md — the "⚠️ While the upstream sync is active, do not hand-edit modules/" warning is inverted: modules/ is now authored here. Also drops the sync/manifest.yml bookkeeping step from Dropping a version, drops the manifest repointing from the release-rollover note, and marks the branch references in the Versions imported table as historical.

Consequence worth stating explicitly

This stops mirroring for all seven products, not just ocis. main, server, webui, desktop, ios and android content freezes at its current state; any future commit in those upstream repos will no longer reach this repo. Those repos have no open issues, so nothing is being dropped on the floor — but authoring for them now has to happen here too.

Follow-ups in flight

Two draft PRs port the unmerged ocis content that this cutover unblocks (owncloud/docs-ocis#1286 and #1285 — neither ever merged upstream, so no version folder carries their content). They depend on this PR landing first, otherwise a sync run would wipe the hand-edited modules/.

Verification

• bash sync/sync-repo.sh docs-ocis → refuses, exit 1.
• grep -n sync README.md → only the three retired/historical references remain.
• Content unchanged, so the Antora build and npm test are unaffected.

Refs: #44