Add some basic docs for Foldable

[philwills]

@adelbertc's comment in #460 made me realise that Foldable doesn't currently have any docs.

Once again, these are largely based off the Scaladoc, with a few examples added.

This was based on the branch for #460, but if for some reason, you don't want that and do want this, I'm happy to rebase.

[adelbertc]

Could you add a section on a the monoid for product/tuple types? Not sure if you should put it on Foldable or Monoid or both (maybe mention it in Monoid and then give an example in Foldable) - its fairly common (esp. in data processing contexts) to be able to compute multiple "summary" values over a large set of data in a single pass (think streaming data).

[non]

This is great, thanks!

I have an ask which is smaller than @adelbertc's: we should make sure to point out that the built-in signature of List#foldRight is different than Cats'. For now this could just be a sentence but I think it's worth mentioning.

That all said, I'm also happy to merge this now and then update the document in future PRs. 👍

[codecov-io]

Current coverage is 62.90%

Merging #473 into master will increase coverage by +0.12% as of e959c39

@@            master   #473   diff @@
=====================================
  Files          154    154       
  Stmts         2359   2359       
  Branches        66     66       
  Methods          0      0       
=====================================
+ Hit           1481   1484     +3
  Partial          0      0       
+ Missed         878    875     -3

Review entire Coverage Diff as of e959c39

Powered by Codecov. Updated on successful CI builds.

[non]

👍

I'd be happy to merge this now and expand on it later. Thanks @philwills!

[fthomas]

Could you replace "typeclass" with "type class" since we have recently agreed on that spelling (#441).

Other than that, LGTMT and I'm also fine with merging it now.

[ceedubs]

It looks like this PR includes the same monoid doc that is in #460. Is that an accident?

It would probably be good to add some clarification that "Right" vs "Left" is really about association as opposed to starting at the end vs beginning of the data structure. That is, we can implement exists, forall, etc in terms of foldRight on an infinite stream, and they can still terminate (depending on the values in the stream). However, with documentation I'm usually pretty inclined to get something out there and people can expand upon it over time, so I give a 👍 for this once the monoid thing is sorted out.

[philwills]

@ceedubs I've rebased against master, so that this is clean of the changes from #460.

I'm not sure I understand the distinction around right/left being about association well enough to write about it yet, but might do some more digging/thinking, then give it a go.

[ceedubs]

The Travis error is unrelated.

I can't speak for the CSS change, but the rest looks good to me.