[doc] New hardware vendor getting started

[balopat]

It would be great to have a doc that lists all the different things a hardware provider can and should do when integrating with Cirq. Namely,

• write an RFC
• documentation (follow the notebook guidelines)
  • access and auth (if available)
  • getting started guide
• integration process (external vs internal module)
• design - interfaces to implement (Sampler, Device, ...)
  • create device abstraction / operation validation for supported gateset
  • noise model
  • come up with serialization (or use cirq.to_json)
  • ensure Simulators work (built-in, qsim)
  • integrate service (if available)

and more...

Related #3245.

[dstrain115]

Discussion from cirq cync: let's still do this, but it can be done after 0.15