The XCM documentation by Parity was announced in this forum post: Introducing the XCM docs and has been hosted on XCM: Cross-Consensus Messaging - XCM Documentation.
With the objective of having a central place for XCM, and the increasing XCM resources in the polkadot wiki, the docs have been migrated to the wiki: XCM Documentation · Polkadot Wiki.
All future developments of the docs will be done in the wiki.