[mlir][NFC] Clean and Fix broken Affine documentation

- Modified single-quote to back-quote at op name, etc.
- Remove a duplicated `affine.store` op's doc
- Fix broken links
- Move Syntax of `StoreOp` and `LoadOp` from Affine.md to AffineOps.td

Reviewed By: bondhugula, dcaballe

Differential Revision: https://reviews.llvm.org/D142858