Add citation page to website

[dmcdougall]

Does what it says on the tin. I fluffed up earlier and accidentally committed to the repository (0ec706f) using github's interactive editing feature (I wanted to add this without cloning the whole site because it's big). I reverted my fluff (19c7998) and made this PR so we have a paper trail of contributions.

Sorry for any confusion.

Please give feedback on what I've written. I am writing a paper and wanted to reference matplotlib in it since all my figures were produced with matplotlib. I realised we hadn't actually added the citation stuff to the site. I shamelessly stole the ideas/layout from @fperez.

Thanks.

[dmcdougall]

And for some reason the files I edited didn't end with \n, so I've added those in. Thanks.

[mdboom]

The website is entirely generated from the docs in the matplotlib source tree, so changes should be made there, and then I just build and push to repository.

I think the file you want to edit is doc/_templates/layout.html which should add the citation link to every page, not just the handful in this PR.

Does that make sense? I'm going to go ahead and close this PR.

[dmcdougall]

@mdboom Oh man! That makes so much more sense! Thanks.