[website] Add Toolpad core landing page

[prakhargupta1]

Preview: https://deploy-preview-3690--mui-toolpad-docs.netlify.app/toolpad/

[prakhargupta1]

@Janpot In the features section of https://deploy-preview-3690--mui-toolpad-docs.netlify.app/toolpad/:

1. I tried showcasing dashboard layout component like its done here: https://mui.com/toolpad/core/react-dashboard-layout/. Instead of coming in the iframe it comes over the entire page.
2. Notifications snackbar throws some error.

Can you have a look please?

[Janpot]

1. There doesn't seem to be an iframe, so we'll have to wrap the component with something like https://github.com/mui/material-ui/blob/d03b6af3a2789af3c55db59f2991205a1f854103/docs/src/modules/components/DemoSandbox.js#L95. I won't have time to look into this before my holidays.
2. If you're not using the Toolpad app provider, you'll have to wrap your page with a <NotificationsProvider> as per docs.

[prakhargupta1]

1. There doesn't seem to be an iframe, so we'll have to wrap the component with something like https://github.com/mui/material-ui/blob/d03b6af3a2789af3c55db59f2991205a1f854103/docs/src/modules/components/DemoSandbox.js#L95. I won't have time to look into this before my holidays.

Thanks for the direction, I tried adding it, but couldn't make it work.

[apedroferreira]

1. I tried showcasing dashboard layout component like its done here: https://mui.com/toolpad/core/react-dashboard-layout/. Instead of coming in the iframe it comes over the entire page.

Can you have a look please?

Taking a look at this. Added a fix, just needed the iframe prop in DemoSandbox!

[Janpot]

Fine to merge this and fill in the blank demos later (page container, authentication)