Documentation: more gn, less gyp

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4698
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=4698

Change-Id: I03100542752a769060a7f0c9671cc44acbea2e48
Reviewed-on: https://skia-review.googlesource.com/4698
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Hal Canary <halcanary@google.com>
6 files changed