Add info about neorg presets

[vhyrro]

Had a lil bit of time so patched this PR together, I hope you're okay with its length and stuff! Since Neorg presets aren't really discussed anywhere in the docs I thought having some info about them here would be a good idea :)

Lemme know if you want me to add/change/take away anything.

[vhyrro]

I'm thinking of other things we could add that are like very special. Neorg has a lot of stuff but basically all of it is fairly widely known :P

Perhaps the presenter module which allows for making presentations in Neovim? I don't think many people know about that one lol