docs: add add-on docs

[manuel3108]

Closes #241

I have only started writing the docs for drizzle so that we can discuss what we want to show in the docs and what we don't. This should also be a great opportunity to test the docs preview generation.

Additional thoughts:

• Is there a way / wish / need to create those docs pages automatically? We already have most of the information inside the add-on, but i don't see a way how we could add those texts in between to the adder. I think the manual way is the best way at the moment.
• I was thinking about including the add-on logos here but i did not find a proper way to make this work. Other sites like the astro integrations don't use logos as well. If we don't want to use them , we should delete the logos in this pr.

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[Rich-Harris]

preview: https://svelte-dev-git-preview-cli-272-svelte.vercel.app/

this is an automated message

[benmccann]

headless with a head? lol. I just checked and that's their tagline, but it's completely unintelligible, so I think we should write our own description for it 😄