Organize API Docs

Would you accept a PR to organize the API docs a bit better?

Culori is great, but I find the docs to be pretty difficult to use, and it limits my ability to use the library. The main problems:
- The function list is in alphabetical order instead of grouped logically. eg. `blerp`, `lerp` and `unlerp` are not grouped together, so if you're unfamiliar with the library, you might overlook `blerp` and `unlerp`. I did. And it makes it really difficult to scan and get a quick sense of what's culori can do.
- Since the list of functions is at the top of the page and they link to anchors, you're always jumping back and forth, losing context.

I'd be happy to organize it so it's easier to use, but I figured I'd ask first before doing the work and putting up a PR. I think that having the functions in a fixed sidebar and grouping them logically would help a lot. I looked at the docs code and it seems like a straightforward change.