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

sidecar/src/shard.rs

@@ -2,7 +2,7 @@
 //!
 //! The shard is the TCP *client*: it dials out to us. So the sidecar owns the listener, and the
 //! shard's outbound socket is the only thing that ever connects. This is the whole reason the game
-//! is never directly reachable from the website — it exposes no port. See docs/PLAN.md §2.
+//! is never directly reachable from the website — it exposes no port. See https://gitea.whitlocktech.com/RunicGateway/docs/src/branch/main/link/PLAN.md §2.
 //!
 //! Framing is newline-delimited JSON, bidirectional: the shard sends events, we send commands. We
 //! accept one shard connection at a time and re-accept when it drops (the shard reconnects on its