Documenting air coding practices

[erwei-xilinx]

Users need to be provided with a documentation on the scheduling implications of input IRs, in order to facilitate hand-written AIR designs using AIR dialect's python bindings.

This issue is intended to keep track of MLIR IR patterns that AIR can properly handle as input, and how AIR compiler interprets them as it generates physical IRs for downstream compilers.

[erwei-xilinx]

TODO: code pattern missing documentation. #765