update canvaskit docs to new API

Also add a dynamic text shaping demo

No-Try: true
Docs-Preview: https://skia.org/?cl=195440
Bug: skia:
Change-Id: I1e220cecae080a6a643fed8a1b9771ba8fe60915
Reviewed-on: https://skia-review.googlesource.com/c/195440
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
2 files changed