[docs] Explain integration with existing apps better on the docs introduction

[bharatkashyap]

• People will be coming from the Material UI docs, but our Installation/Setup instructions are addressing users more from the create apps from scratch PoV. We should add focused content on users who have existing apps.

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[prakhargupta1]

For this guide, would it be a good idea to show how to add Toolpad core in the app-router and page-router templates available here: https://mui.com/material-ui/getting-started/example-projects/ ?

[Janpot]

Backlog grooming Brainstorm:

• Add section at the top of demo pages. "If this is your first time on this page => => link to page explaining Toolpad core scope"
• On installation page, move "manual installation" to the top.
• Add a page that explains basic concepts. Like https://mui.com/x/react-date-pickers/base-concepts/
  • "We don't do routing, we integrate with different routers."
  • Explain there is a hierarchy between app provider and the other components. Acts as bridge between your runtime and Toolpad components
  • imports https://mui.com/x/react-date-pickers/base-concepts/#imports
  • Slots https://mui.com/x/react-date-pickers/base-concepts/#overriding-slots-and-slot-props
  • localization story [let'd do this later]

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