Create a "How-To Get the Availability Zones in a Region" section in the docs

[Doug-AWS]

No description provided.

[Doug-AWS]

Here is what I have so far:

You can get the AZs in the current region by using the
::py:class:_aws-cdk_core.AvailabilityZoneProvider class,
as shown in the following example.

.. code-block:: js

const zones: string[] = new AvailabilityZoneProvider(this).availabilityZones;

for (let zone of zones) {
// do somethning for each zone!
}

[eladb]

Yes, looks right. I think we should add some more details (or reference) here on how contextual providers work.

Also, there's a nuance (pending #227): when this is executed outside of the context of a toolkit synthesis (i.e. in unit test), the provider returns dummy values. This is usually quite fine, but people should be aware and not surprised by this.

[Doug-AWS]

Done. Since we already had this information in the Environmental Context topic, I re-used that content.

This info is in the "How-To: Get the List of Availibility Zones" section of cookbook.rst in the dougsch/addhowto branch. Once I've finished this round of how-to sections, I'll open a PR and add the PR # to this issue and close it.

[Doug-AWS]

Done