update doc index

Add SkRegion, SkTextBlob, SkTextBuildBuilder
to top level API documentation

TBR=jcgregorio@google.com

No-Try: true
Docs-Preview: https://skia.org/?cl=147814
Bug: skia:6898
Change-Id: Ie4033cd7e542b2e462521aa5c838bb2457d32300
Reviewed-on: https://skia-review.googlesource.com/147814
Commit-Queue: Cary Clark <caryclark@skia.org>
Reviewed-by: Cary Clark <caryclark@skia.org>
2 files changed