Draft: WIP Implement a landing page for the documentation

This is a work-in-progress implementation of the landing page that was designed during the documentation sprint in November 2022.

There is probably further work to be done on the copy. We could also adjust the image colors to match Software Heritage visual identity.

• Supersedes: !238 (closed)
• Closes: #3867 (closed)

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Migrated from D8894 (view on Phabricator)