Document RAM support

[proppy]

#861 has made enough progress that we should start documenting the RAM support properly.

https://github.com/google/xls/blob/main/xls/examples/delay.x can be used as the main example.

Ideally we would also demonstrate end-to-end integration with RAM macros with opensource PDKS (ASAP7, GF180MCU, SKY130).

/cc @grebe

[proppy]

@rdob-ant might also have some good input on what to include there (because of their work on #1038)