Split API docs into separate pages, remove redundancy with kernels/transforms

[st--]

Maybe one could also just move the list of transforms here (the full API section is a bit overwhelming to be honest). But if we want to do this, it might best to change it in a separate PR and maybe for some other sections as well.

Originally posted by @devmotion in #213 (comment)

[devmotion]

Fixed by #236.