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/BridgePresence.cs

@@ -6,7 +6,7 @@ using Server.Mobiles;
 namespace Server.Custom.Bridge
 {
     /// <summary>
-    ///     The presence stream (docs/PROTOCOL_2.md §11 #1/#2): who is online and where. Two parts:
+    ///     The presence stream (https://gitea.whitlocktech.com/RunicGateway/docs/src/branch/main/link/PROTOCOL_2.md §11 #1/#2): who is online and where. Two parts:
     ///
     ///       presence.online - a periodic population snapshot (total, per-facet, per-region), emitted
     ///                         on a sweep but only when it changes, so the site has a live "N online"