Initial release of the Quantization dialect

Includes a draft of documentation for the quantization setup.

Given how many comments such docs have garnered in the past, I've biased towards a lightly edited first-draft so that people can argue about terminology, approach and structure without having spent too much time on it.

Note that the sections under "Uniform quantization" were cribbed nearly verbatim from internal documentation that Daniel wrote.

PiperOrigin-RevId: 241768668