Update free monad docs

[ceedubs]

This resolves #804.

I also updated the "pure" compiler to use State. It previously had
said that a pure compiler couldn't be written and used with foldMap,
but that's not really true if you use something like State. I think
this is probably a cleaner and more useful solution.

@mandubian @raulraja would you be available to review?

[codecov-io]

Current coverage is 89.26%

Merging #832 into master will decrease coverage by -0.04% as of 69c4737

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- Missed         248     249     +1

Review entire Coverage Diff as of 69c4737

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[raulraja]

An alternative to liftF is to use Free.inject when targeting cats.data.Coproduct as a composition mechanism maybe it should be mentioned here.

[raulraja]

I guess it's mentioned below. Perhaps we should consider adding some kind of TOC at the beginning of each tutorial to show at a glance the different sections and topics being described.

[raulraja]

@ceedubs LGTM!. My comments are just suggestion for future consideration in case the tutorials become too long.

[non]

👍

[mandubian]

yes that was required! thanks for the effort ;)
👍

[ceedubs]

Thanks for the quick feedback everyone. @raulraja I agree that tables of contents might be helpful in docs. I'll create a separate issue to track that.