Fix Kmeans quantization randomness, Explicit documentation on resize

[opeyemibami]

What kind of changes does this PR introduce?

1. Fix: Fix randomness in Kmeans quantization mode by setting the random_state to 2024
2. Doc: Explicitly state the resize shape (256 x 256) used by pylette under the hood for clarity purposes
3. Doc: state median-cut implementation alongside Kmeans for clarification on the future plan list

What is the current behavior?

1. For every code run, the frequency, luminance, and palette colors keep changing when mode='KM'
2. It can be confusing to users who are not great at inspecting code functionality
3. It can be confusing to users who prefer direct documentation of available features

What is the new behavior?

1. For every code run, the frequency, luminance, and palette colors are constant when mode='KM' as expected
2. More explicit to users who are not great at inspecting code functionality
3. More explicit to users who prefer direct documentation of available features