[CanvasKit] Add docs

No-Try: true
Docs-Preview: https://skia.org/?cl=155400
Bug: skia:
Change-Id: I08b14ffa569b2bff81287f854c493a4cedabb5bb
Reviewed-on: https://skia-review.googlesource.com/155400
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
1 file changed