[docs] Product onboarding content architecture

[oliviertassinari]

Summary

I have been pushing lately for the following docs architecture, taking this from the angle of customers:

• Marketing page: "I’m not sure I need this product. Could it have the potential to solve my problem? Sell it to me. I’m not a user yet".
  Those pages are shown at the root URL, e.g. https://mui.com/x/react-data-grid/, https://mui.com/x/react-charts/ but use the same layout as the marketing page, e.g. https://mui.com/x/. They don't use the layout of the docs. I think it's really important because those pages in the docs are annoying from the perspective of a regular user, at least, it's how I experienced https://mui.com/x/react-date-pickers/ so far, pain.
• Overview page: "Which component from this product should I use for my problem? I’m already sold on it, I’m already using it".
  If the content requires depth to pick the right component, e.g. pickers for birthday, chart type based on what needs to be shown https://flourish.studio/blog/choosing-the-right-visualisation/, then it might not be wise to have the depth in the overview page, but best to move it to a guide page.
• Demo page: "I need further help coming from the marketing or overview page. Can you give me real examples?".
  This page is not as detailed as the docs, so not as overwhelming, but goes more into the details than the marketing or overview page, when you want to learn more to either see proofs or figure out what you need.
• Quickstart page / Getting started: "I want to use this but how do I actually do it?".

I'm curious about collecting thoughts on this. What if we were to execute it?

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cc @mapache-salvaje