[docs] Add placeholders for upcoming features

[prakhargupta1]

Preview: https://deploy-preview-4175--mui-toolpad-docs.netlify.app/toolpad/core/react-notification-center/

[bharatkashyap]

I think the idea was to add individual pages with links to the issue from each page, with a standard work in progress warning callout on each of these pages. The benefit would be that the features would show up in the navigation instead of the roadmap page which might not be viewed by as many people. Not sure if I understood incorrectly.

Benchmark: https://mui.com/x/react-data-grid/server-side-data/aggregation/

[prakhargupta1]

I thought adding 9-10 items as separate pages in the side nav would be too much noise for the readers. I think they should just be links to the issues grouped together on a single page. We can put this group in multiple places:

1. Overview page
2. All Components page
3. As a separate category of 'Upcoming features' as the fourth item on the main list:
   • Introduction
   • Components
   • Upcoming features (Here, if needed for each of them we can add a few lines explaining them)
   • APIs

[bharatkashyap]

I thought adding 9-10 items as separate pages in the side nav would be too much noise for the readers. I think they should just be links to the issues grouped together on a single page. We can put this group in multiple places:

1. Overview page

2. All Components page

3. As a separate category of 'Upcoming features' as the fourth item on the main list:

   • Introduction
   • Components
   • Upcoming features (Here, if needed for each of them we can add a few lines explaining them)
   • APIs

Perhaps not all, but I definitely think these items are only discoverable if they are in the navigation sidebar; otherwise the discoverability of these is quite low in my opinion, might not help us attract as much attention as we want. Let me know what you think

[Janpot]

Backlog grooming proposal:

• likely not enough visibility in this section
• remove the section in the introduction page
• put it in navigation as items (us "planned" chip as X) + placeholder pages like https://mui.com/x/react-charts/gantt/
• retain the following:
  • data section with
    • CRUD
    • stat card
  • under authentication section
    • Sign-up page
    • RBAC components
  • under layout
    • notification center