ipns: ensure old links work

Gently point at new location (until we clean up docs.ipfs.tech)

Author: lidel

IPNS.md

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+Moved to [./ipns](./ipns/)

ipns/README.md

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+# IPNS Specifications
+
+## About
+
+**This directory** contains specifications related to IPNS.
+
+## **Intended audience**
+
+The goal of this spec is to provide the reference documentation that is
+independent of specific language or existing implementation, allowing everyone
+to create a compatible IPNS Record Publisher or Resolver.
+
+# Specification index
+
+* [IPNS.md](./IPNS.md) ← **START HERE**
+  * [IPNS_PUBSUB.md](./IPNS.md) - IPNS over PubSub