Sharding: Refer to XMover utility

[amotl]

Please review the patches enumerated here, so we can release the XMover utility and mention it here. It is a perfect fit for extending the document to guide readers into applying excellent tooling.

• Looking-glass utilities for exploring what's going on: CrateDB shard analysis and beyond cratedb-toolkit#513 (comment)

Originally posted by @amotl in #355 (comment)