Add logos of our users to our homepage

[ericholscher]

Fixes https://github.com/readthedocs/website/issues/285

Not 100% sure the best way to make these line up. Is there a standard way?

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📚 Documentation preview 📚: https://readthedocs-about--298.org.readthedocs.build/

[ericholscher]

With these changes, I think it looks shippable:

We can definitely iterate on it, but I think it definitely improves the landing page already.

[ericholscher]

@agjohnson Would love another look here, would be good to get it shipped!

[ericholscher]

Got it working:

[ericholscher]

🎉