Specify deprecation policy in the docs

Since https://github.com/dwavesystems/dimod/pull/1135 we remove deprecated features two minor version releases after the deprecation. So if a feature was deprecated in 0.11.2, it will be removed in 0.13.0. This is similar to [NumPy's deprecation policy](https://numpy.org/neps/nep-0023-backwards-compatibility.html#nep23) although we do not release on a timed cycle like they do.

Note that for projects that require binary compatibility, they should not assume that backwards compatibility will be maintained in the same way. For these projects they should only allow minor versions they test with.

This should be specified in the docs.

See also https://github.com/dwavesystems/dwave-ocean-sdk though note that the SDK (as of March 2022) follows true semantic versioning.