[sk] Add documentation

Change-Id: Ibf8cad25af60f61541ebe064a27d235dceea442b
No-Try: true
Docs-Preview: https://skia.org/docs/dev/tools/sk?cl=397037
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/397037
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
1 file changed