Add twitter cards

[jarrodmillman]

See numpy/numpy.org#588.

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[jarrodmillman]

I am not sure if this worked. Before

Clearly, no card found. Now, we get

It says the card was loaded successfully, but it is still "Unable to render Card preview". The source now includes

    <meta name="twitter:card" content="summary_large_image"/>
<meta name="twitter:image" content="https://theme.scientific-python.org/images/logo.svg"/>

<meta name="twitter:title" content="Scientific Python Hugo Theme documentation"/>
<meta name="twitter:description" content="Documentation &amp; template site for the Scientific Python Hugo Theme."/>

which seems correct.

I will merge and see if maybe it is an issue with it being a preview somehow...