first cut at SkRect, SkIRect documentation

All functions should have descriptions, examples, cross-references.
References and spelling have been checked.
More work to do creating and organizing topics.

Docs-Preview: https://skia.org/?cl=56140
Tbr: caryclark@google.com
Bug: skia:6898
Change-Id: I9d1e55d04ab64874c33cac8b91534aa192c2f545
Reviewed-on: https://skia-review.googlesource.com/56140
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
9 files changed