Website: add canonical documentation which can be reused by all implementations

[boronine]

Suggested by @mity here: hsluv/hsluv-c#6 (comment)

[mity]

This should imho be covered by such documentation:

• All input parameters, their meaning and expected ranges.
• All return values, their meaning and expected ranges.
• A note HPLuv color space covers only a subset of all possible RGB triplets.
• (For RgbToHpluv) How to correctly detect the input RGB falls outside the HPLuv space. (Have all the three return values to be checked or is e.g. just checking the saturation falls in 0...100 good enough?)
• (For RgbToHpluv) For any RGB input outside the HPLuv color space, how to get "the closest" valid HPLuv color, in some reasonable sense. (Does e.g. clamping one or more values as returned by the function RgbToHsluv into the valid HPLuv colorspace work?)

[boronine]

Just wanted to thank @mity for writing this up and leave my approval. I encourage anyone to take a stab at this. If left open, I might do it myself when I get some free time this year.

[boronine]

HPLuv clamping issue encountered here: #78