[PathKit] Add docs page

No-Try: true
Docs-Preview: https://skia.org/?cl=147620
Bug: skia:8216
Change-Id: Iec5d1be49504b579bdc0893658941d3d722f676c
Reviewed-on: https://skia-review.googlesource.com/147620
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
2 files changed