[llvm] [mlir] [NFC][mlir][mesh,shard] Fixing misnomers in mesh dialect, renaming 'mesh' dialect to 'shard' (PR #150177)

[Frank Schlimbach via llvm-commits]

fschlimb wrote:

I updated Shard.md to address @joker-eph 's concerns above. 

Further fixes of doc strings etc can be done in a separate PR.

https://github.com/llvm/llvm-project/pull/150177