docs: Add instructions on how to download isolated intermediaries.

Screenshot:
  https://screenshot.googleplex.com/2cXAQMqypHv.png

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2316113002
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=2316113002

Review-Url: https://codereview.chromium.org/2316113002
3 files changed