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/BridgeSweeps.cs

@@ -11,7 +11,7 @@ namespace Server.Custom.Bridge
     /// <summary>
     ///     The three polled streams, for state that has no EventSink: player vitals, house decay,
     ///     and money supply. All three run on the Core thread via repeating Timers, and the
-    ///     measured cost (docs/PLAN.md §1) is why they can: at the seeded scale a full pass of all
+    ///     measured cost (https://gitea.whitlocktech.com/RunicGateway/docs/src/branch/main/link/PLAN.md §1) is why they can: at the seeded scale a full pass of all
     ///     three is well under a millisecond.
     ///
     ///     Timers do not fire during a world save (Timer.cs:322), so a sweep that would have landed