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/BridgeSocial.cs

@@ -7,7 +7,7 @@ using Server.Guilds;
 namespace Server.Custom.Bridge
 {
     /// <summary>
-    ///     The guild stream (docs/PROTOCOL_2.md §10.1). Guilds have almost no useful EventSink:
+    ///     The guild stream (https://gitea.whitlocktech.com/RunicGateway/docs/src/branch/main/link/PROTOCOL_2.md §10.1). Guilds have almost no useful EventSink:
     ///     EventSink.CreateGuild is only the load-time deserialization factory (Server/World.cs), and
     ///     leave/disband/leader/alliance changes raise nothing. Only EventSink.JoinGuild is real. So,
     ///     exactly like <see cref="BridgeChamps"/>, the roster is polled: enumerate BaseGuild.List each