[Documentation] Update RGBa parameter example to a perceptual colorspace like CIELab?

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In the [documentation about parameters](https://umap-learn.readthedocs.io/en/latest/parameters.html), 4-dimensional RGBa color coordinates are visualized in 2D, and colored, to showcase how various UMAP parameters affect the final reduction. However, RGBa is not a perceptual color space, which means readers' intuitions about which points should be grouped together based on color similarity will be off.

One could instead convert the RGBa coordinates to CIELab (a perceptual colorspace). One way to do this is to go from RGBa to RGB, and then RGB to CIELab (a perceptual color space) via the color transformation functions found here: https://scikit-image.org/docs/dev/api/skimage.color.html. There might be other ways to do this conversion as well.

Start with the parameters documentation linked in the issue and inspect the RGBa visualization example. Read the scikit-image color transformation documentation to compare the proposed RGB-to-CIELab route with available alternatives. Done means the example uses an appropriate perceptual colorspace and the rendered documentation remains clear.

Written by the indexing model from the issue text.