Document the 2pt conical gradient

I've also updated/simplified our source code
to match the document.

Please ignore everything except the md and cpp file.

See the rendered document in:
https://skia.org/dev/design/conical?cl=89340

If the math is not rendered properly, refresh the page.

No-Try: true
Docs-Preview: https://skia.org/?cl=89340
Bug: skia:
Change-Id: I9b9306c1979960ccec0d3ab833391c649edb833c
Reviewed-on: https://skia-review.googlesource.com/89340
Commit-Queue: Yuqian Li <liyuqian@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
21 files changed