add link to old docs

src/introduction/introduction.md

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 # Introduction
 
+> **_NOTE:_** This book is a work in progress. To get a fuller picture of wormhole, also check out [a previous version of these documents](https://docs.wormholenetwork.com/wormhole/).
+
 Welcome to the xDapp Book and beginning your journey to be a cross-chain developer. As crypto ecosystems mature, more and more applications will need to interact with programs outside the infrastructure they are deployed on. In Web2 we see this every day, as infrastructure like Azure, GCP, and AWS is abstracted away from the everyday user and applications can freely talk to each other over HTTP.
 
 For blockchain applications to unlock their true potential, they too must be able to talk to applications on various infrastructure using a common language. These cross chain applications (xDapps) give rise to cross chain data (xData) and cross chain assets (xAssets). With these powerful primitives, blockchain applications can deliver on their promise of interoperability and composability.