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/BridgeHousing.cs

@@ -7,7 +7,7 @@ using Server.Multis;
 namespace Server.Custom.Bridge
 {
     /// <summary>
-    ///     The housing registry (docs/PROTOCOL_2.md §11 #9). BridgeSweeps already emits house.decay
+    ///     The housing registry (https://gitea.whitlocktech.com/RunicGateway/docs/src/branch/main/link/PROTOCOL_2.md §11 #9). BridgeSweeps already emits house.decay
     ///     *transitions*; this is the complementary *board*: one row per house with owner, location,
     ///     region, co-owners, value, and current decay level, so the website can render an owner→houses
     ///     map. Like the other Part B boards it is a diff sweep over BaseHouse.AllHouses — emit