Start updating iOS docs.

Start updating iOS docs.

This removes the docs for the old GYP-based local development and
replaces them with ones for GN-based development.

I have not yet updated the docs for iOS on the bots... still in flux.

Change-Id: I1f9c5c1a3331ae192dc08c614ef7da26924f808e
Reviewed-on: https://skia-review.googlesource.com/8002
Reviewed-by: Stephan Altmueller <stephana@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>