main: branch-aware README + BRANCHES.md + versions.json

Rewrites main as a docs-only landing branch. Adds:
- BRANCHES.md — per-branch chart (client build, status, notes)
- versions.json — machine-readable manifest the Easy launcher reads
- Branch-picking step + Easy-app/Docker URLs updated in README +
  docs/advanced-setup.md

Drops the per-folder README stubs (server/, admin-ui/, client-mod/,
docker/, easy-app/, tools/) — the actual code lives on per-version
branches now, not on main.

BRANCHES.md

@@ -0,0 +1,62 @@
+# Branches
+
+DorkNet maintains a long-lived branch per supported Rec Room client
+build. Each branch is an independent fork of the server + client patcher
+tuned to that build's wire protocol. Pick the one matching your install.
+
+## Active branches
+
+### `december-2020-12-18`
+
+- **Client build:** Rec Room 2020.12.18
+- **Status:** active
+- **Notes:**
+  - Single EF Initial migration; SQLite + Postgres both work out of the
+    box.
+  - Includes the chat-threads, clubs, playlists, weekly-challenge, and
+    room-keys subsystems added late-2020.
+  - Server's `Versions/Late2020VersionPlugin` is the active version
+    plugin; client header `X-DorkNet-Version: december_2020_12_18`.
+
+### `march-2020-03-10`
+
+- **Client build:** Rec Room 2020.03.10
+- **Status:** pending import
+- **Notes:**
+  - Earlier wire shape — no clubs, no playlists, simpler store catalog.
+  - Diverges from `december-2020-12-18` by ~170 files; cannot be
+    unified with the plugin abstraction alone.
+
+Setup for either is the same — see [docs/advanced-setup.md](docs/advanced-setup.md)
+(starts with a "pick your branch" step).
+
+## How to pick
+
+If you don't know which Rec Room build you have, check:
+
+- **Steam**: right-click *Rec Room* → Properties → Local files →
+  Browse → look at the `Recroom_Release_Data/StreamingAssets/version.txt`
+  if present, or note the install timestamp.
+- **Manual install** (the typical DorkNet setup path): the build is in
+  the folder name (`RecRoom-2020.12.18`, `RecRoom-2020.03.10`, etc.).
+- **Easy launcher**: auto-detects the build and picks the branch.
+
+## Machine-readable manifest
+
+[`versions.json`](versions.json) carries the same information in a
+format the Easy launcher and install scripts can consume directly. Add
+new versions there too when a new branch lands.
+
+## Adding a new version branch
+
+1. Land the server code on a new orphan branch named `<month>-<build>`
+   (e.g. `august-2021-08-04`).
+2. Add a row to the active-branches list above.
+3. Add the matching entry to `versions.json`.
+4. Add a `docs/<month>-setup.md` setup guide.
+5. Tag a release on the new branch so the Easy launcher's
+   release-fetcher picks it up.
+
+Branches are intentionally long-lived — once a build is supported, its
+branch keeps getting patches against the same wire protocol. Don't
+rebase across branches; cherry-pick when a fix applies to multiple.