docs: move design docs to RunicGateway/docs

Extracted BACKEND_DESIGN.md, HERO_EDITOR.md, and WIKI_UPGRADE.md into the
central RunicGateway/docs repo (under docs website/, full history preserved
via git filter-repo). Repoint the README's two BACKEND_DESIGN.md links at
the new location.

Docs repo: https://gitea.whitlocktech.com/RunicGateway/docs

README.md

@@ -8,7 +8,7 @@ shard — a full-stack app in one repo:
 - **Deploy** — Docker Compose (app + MariaDB) behind a Pangolin reverse proxy. Express serves the built SPA in production.
 - **Shard link** — a live bridge to the in-game ServUO shard through the **uo-link** sidecar ([UOM/link](https://gitea.whitlocktech.com/UOM/link)): the site ingests a live event feed and makes server-side REST calls to show shard status, economy, staff presence, IDOCs, live activity, and per-character sheets. See [Shard integration (uo-link)](#shard-integration-uo-link).
 
-The design reference is [BACKEND_DESIGN.md](BACKEND_DESIGN.md) (API contract, schema, security).
+The design reference is [BACKEND_DESIGN.md](https://gitea.whitlocktech.com/RunicGateway/docs/src/branch/main/website/BACKEND_DESIGN.md) (API contract, schema, security), in the [**RunicGateway/docs**](https://gitea.whitlocktech.com/RunicGateway/docs) repo — where all project documentation now lives.
 
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@@ -229,7 +229,7 @@ npm start               # node server → serves API + SPA at http://localhost:3
 
 Post categories (URL form): `news`, `five-on-friday`, `newsletter`, `screenshots`.
 `authMethod` on a session ∈ `local · totp · mobile · google · discord · oidc`.
-See [BACKEND_DESIGN.md](BACKEND_DESIGN.md) §4 for the full contract, or the interactive Swagger
+See [BACKEND_DESIGN.md](https://gitea.whitlocktech.com/RunicGateway/docs/src/branch/main/website/BACKEND_DESIGN.md) §4 for the full contract, or the interactive Swagger
 docs below for a per-endpoint reference (parameters, request bodies, response codes).
 
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