docs: Reorganize instructions on using a dev server.
docs: Reorganize instructions on using a dev server.

This makes the structure somewhat more explicit, and hopefully easier
to follow what each step is doing and to see one's progress.  It also
provides a more natural framework for the new material we'll be adding
from #2697 and #2715.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile