Move documentation to use the embedded fiddle elements.

Move documentation to use the embedded fiddle elements.

Now code and images will always be in sync, and
users can edit and run the fiddles that are inline with
the docs.

Previews:
  https://skia.org/user/api/skcanvas?cl=8507
  https://skia.org/user/api/skpaint?cl=8507

BUG=skia:

NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=8507

Change-Id: Iaf62d98d9aa73cf7b4e4a6baa522402ecb9e505a
Reviewed-on: https://skia-review.googlesource.com/8507
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>