Trim down quick start docs for a GN world.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4431
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=4431

Change-Id: I2a10494158e7415ae655baf212a951afd87dcbde
Reviewed-on: https://skia-review.googlesource.com/4431
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
11 files changed