Update code block designations

'```mlir' is used to indicate the code block is MLIR code/should use MLIR syntax
highlighting, while '{.mlir}' was a markdown extension that used a style file
to color the background differently of the code block. The background color
extension was a custom one that we can retire given we have syntax
highlighting.

Also change '```td' to '```tablegen' to match chroma syntax highlighting
designation.

PiperOrigin-RevId: 286222976