Move main classes documentation from Appendix

[jxu]

The main classes (Beatmap, Library, Client, Replay, Collection) should be high level sections in the docs for easier access (when I first used slider it was not clear that they were even documented in the appendix)

[llllllllll]

Would you be interested in opening a PR to add the proposed sections? Maybe we could rename "what is a beatmap" to "beatmap" and then merge that page with "working with Beatmaps" and have some more docs there?

[jxu]

I'll try to get around to it.

[llllllllll]

Thanks!

[jxu]

It makes more sense to me to have the main section be "Beatmap" and "Working with Beatmaps" and class docs to be subsections. I haven't decided yet.