make building docs less complex

[ExpandingMan]

Currently the process for building documentation is significantly more complex than in just about any other Julia package I've ever seen. The result is that making even modest improvements to the docs can sometimes be not only painful but error-prone.

I propose the following changes be made:

• Complete remove dependence on Literate.jl. Having example scripts is great, but this isn't really the place for them. The existing examples are small enough that most of the information they provide can simply be moved to the regular docs.
• Eliminate all of the file-moving shenanigans from docs/make.jl. Right now if you go into docs and start changing stuff, you might be unpleasantly surprised to find that your changes have been overwritten. Contributors should be able to assume that changes they make to docs/src should be reflected in the built docs without having to understand an unorthodox make.jl.
• Refactor the documentation hierarchy. In my opinion some of what's in the "how-to guides" (e.g. extending transducers types) are core functionality and should be introduced as such. I propose expanding what is now simply Home into a few pages, including most of what is now in "how-to" and moving what is currently called "manual" to a dedicated and clearly labeled API reference.

I intend to do a PR that implements this, but figured I'd open this issue first to see if any of this is controversial before I start changing stuff.