Documentation for accessing the different repositories of SKPs

BUG=skia:2306
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1683233002
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=1683233002

Review URL: https://codereview.chromium.org/1683233002
1 file changed