Add a landing page for the documentation

This is implements the landing page that was designed during the
documentation sprint in November 2022.

There is probably further work to be done on the copy, maybe tweak
colors and margins on the cards. Proper support for dark mode would
be nice as well. It’s a bit ugly for now.

The images are coming from <https://undraw.co/>. The license does not
require attribution. Full license text reads as follow:

    Copyright 2022 Katerina Limpitsouni

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    copyright license to download, copy, modify, distribute, perform,
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    creator or unDraw.  This license does not include the right to
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