Document page index handling in QuickGrid

[guardrex]

Fixes #37019

Ilona ... A few years ago, a dev asked how to make the manually-built grid of an old Blazor Server EF Core sample return the user to the grid at the scroll position† where they left. [IIRC, QuickGrid had just come out at the time. The sample that we were discussing didn't use it. We don't even have that sample anymore.]

What I'd like to do in this area is cover page index management for QuickGrid. At the time, I was too busy to flesh out a page index example, but I reached it for work today. As usual, I want to provide a fully working, cut-'n-paste example. How does this look to you?

†BTW ... AFAIK, scroll position isn't something that can be tracked/set with a QuickGrid. I know that such a grid can be built that would do something like that, but I don't think that the built-in QuickGrid component has such a feature baked into it. If I'm wrong about that ... if there would be some way to preserve scroll position for coming back from a details page, please let me know. 👂

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aspnetcore/blazor/components/quickgrid.md	aspnetcore/blazor/components/quickgrid

[Copilot]

Pull request overview

Adds documentation and a cut-and-paste sample showing how to preserve and restore a QuickGrid page index when navigating to a details page and back.

Changes:

• Document PaginationState.CurrentPageIndex and PaginationState.SetCurrentPageIndexAsync for saving/restoring paging state.
• Add Details.razor and Characters.razor example snippets using query string parameters to round-trip the page index.