Document `zulip-api-examples` script.

[roberthoenig]

zulip-api-examples is a little script that prints the path to the examples directory of the installed zulip package.

We should document it in our API docs. At the creation time of this issue, the proper place to add such a documentation (/api/) did not have the right layout adding a section like this (see the discussion here. Once that's resolved, someone can claim this.