Add a link to the skrub-materials website in the main documentation

[rcap107]

Describe the issue linked to the documentation

Now that the skrub-materials website is in an acceptable shape, we should add a link to it in the main documentation.

Suggest a potential alternative/fix

No response

[Neilblaze]

@rcap107 Is this up for grabs? If yes, then I'd take it up.

[jeromedockes]

yes, it is! thanks a lot this will be very useful :)

[Neilblaze]

Cool, I'll create a PR then! Thanks! 😃