Add link to Pangeo on landing page

[brian-rose]

Along with https://github.com/ProjectPythia/projectpythia.github.io/pull/215, this PR finishes addressing https://github.com/ProjectPythia/projectpythia.github.io/issues/214 by acknowledging the Pythia + Pangeo relationship on the Foundations landing page.

Note that I converted the landing page back to markdown, since the notebook wasn't actually computing anything.

If we decide to move forward with https://github.com/ProjectPythia/pythia-foundations/pull/62 (self-generating cover art) at some point, we can always revert back to a notebook for this page. In the mean time, we can enjoy easier edits and GitHub-based reviews.

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[brian-rose]

Oops, found a broken link caused by recent changes to Matplotlib content. Unrelated to the main thrust of this PR, but fixed anyway.

[brian-rose]

pinging @ktyle @clyne @kmpaul: are we happy with this branding for the Foundations cover page?

[kmpaul]

@brian-rose: Looks good to me.