Add SkSL color management docs
Add SkSL color management docs

Explains several different features for:
- Uniforms
- Raw image shaders
- Color transform intrinsics

Bug: skia:10479
Change-Id: I9254d30283e74f039eb1490636efec1ddd125aa2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/492656
Reviewed-by: Michael Ludwig <cb1f3dd24de0f0b611b26bc28bec758a551d9b7d@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia