Reflection illumination White LED part instructions requires more detail

I have built various iterations of the reflection illumination module and case over the past weeks. The biggest challenge I encountered was to improve the image quality, especially reducing vignetting. The most important issues I've encountered were with the LED. Most notably:

1. The clear lens that comes with most LEDs causes non-uniform illumination of the sample.
2. The required luminous intensity matters a lot.

Regarding 1: A diffuse lens LED with a high enough luminous intensity is hard to find. I ended up making the clear lens diffuse with sandpaper, which worked surprisingly well.

Regarding 2: The currently suggested LED (NSPL500DS) is rated 22 cd. In my experience, a stronger LED helps with the image quality. I ended up using the C503D-WAN-CCBEB151 (65 cd).

Adding this information to the documentation for the White LED part information (docs/parts/electronics/white_led) could save people some work in the future.

I'm currently finishing up a merge request to add this information to the docs. I hope you will be able to merge this. I'd be happy to make further improvements to this pull request where needed.