Document classical control

[95-martin-orion]

This PR adds a docs page for classical control detailing some basic use cases and edge behavior to consider.

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[95-martin-orion]

CC @augustehirth and @daxfohl.

For @daxfohl: are there any other weird behaviors / use cases you think are worth highlighting here?

[daxfohl]

Do you want to highlight deferred measurements conversions? There are examples in the tests. https://github.com/quantumlib/Cirq/pull/4849/files. This is allows running equivalent circuits on a real device (at the cost of requiring ancilla qubits).

Should also highlight QASM support once it's fixed. (Victory marked it as post 1.0 but it should be a straightforward fix if anyone has time before then #5388).

[95-martin-orion]

Do you want to highlight deferred measurements conversions?

Done, plus a brief general mention of how transformers interact with classical control. There's something to be said for the fact that transformers are a separate complex topic, but since this will be filed under "Advanced" I think it's reasonable to assume that readers are familiar with (or at least willing to read) the transformers documentation.

Also maybe the teleportation circuit would be good to reference

Copied the example from examples/ into the Basic Conditions section.

[augustehirth]

@95-martin-orion What else needs to be done for this? After your updates it looks great to me.

[95-martin-orion]

@95-martin-orion What else needs to be done for this? After your updates it looks great to me.

Just need approval from maintainers.