docs: move docs to RunicGateway/docs, repoint all references

Extracted docs/ (ADMIN_CONTROLS, INTEGRATION, PLAN, PROTOCOL_2, RESEARCH,
SHARD_PREREQS) into the central RunicGateway/docs repo under link/, with
full commit history preserved via git filter-repo.

The source cites these design docs by section throughout, so every in-repo
reference (C# + Rust comments, Bridge.cfg, and the READMEs) is repointed at
the new docs-repo URL. README references are rendered as markdown links; a
Documentation pointer section is added to the top-level README.

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

overlay/Scripts/Custom/Bridge/BridgeJson.cs

@@ -8,7 +8,7 @@ namespace Server.Custom.Bridge
 {
     /// <summary>
     ///     Outbound JSON is written by hand into a StringBuilder. It runs on the Core thread for
-    ///     every emitted event, and the measured budget in docs/PLAN.md assumes this cost, not a
+    ///     every emitted event, and the measured budget in https://gitea.whitlocktech.com/RunicGateway/docs/src/branch/main/link/PLAN.md assumes this cost, not a
     ///     reflection serializer's.
     ///
     ///     Inbound JSON is parsed with JavaScriptSerializer. Commands arrive at human rates, so