Update Viewer docs.

Also deletes SampleApp docs.

No-Try: true
Docs-Preview: https://skia.org/?cl=74700
Change-Id: Ida594d12db9ea38fdd7d91a920ed1cc7afd04e0d
Reviewed-on: https://skia-review.googlesource.com/74700
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed