Update GN docs to mention is_official_build and ndk_api.

Update GN docs to mention is_official_build and ndk_api.

is_official_build is the closest approximation to what we'd recommend people ship, and we'll keep tweaking it further toward that goal.  I want it to be the first choices people see reading the doc to nudge them to defaulting into that if they don't know better.  The familiar old Debug and Release for developers come later.

It'd be a fine choice, for instance, for fiddle to build against.  I dropped Stripped because that's something is_official_build now does more naturally.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2342333002
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=2342333002

Review-Url: https://codereview.chromium.org/2342333002