Update skia.org docs to reference Gerrit instead of Rietveld

BUG=skia:5958

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4648
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=4648

Change-Id: I6a57b7f9b92f6b769ba2b78f8e0ab9f4ae7bb24e
Reviewed-on: https://skia-review.googlesource.com/4648
Commit-Queue: Ravi Mistry <rmistry@google.com>
Reviewed-by: Heather Miller <hcm@google.com>
4 files changed