Add docs on stateful context pool

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And rearrange the perf docs a little.

@ajcvickers would appreciate your opinion on the scoped context factory pattern shown - we've had discussions around this in the past. The one problem here is that it's not possible to register the custom Scoped factory as IDbContextFactory in DI, since that's already taken by the Singleton pooling factory (can't have the same service registered in both Scoped and Singleton).

Ideally the Singleton pooling factory wouldn't be registered as IDbContextFactory, but rather e.g. as PooledDbContextFactory; this would allow the scoped one to be registered as IDbContextFactory, and injected where multiple contexts are needed. But to do that, users would need to duplicate functionality out of e.g. EntityFrameworkServiceCollectionExtensions.AddPoolingOptions. Nothing here seems particularly bad/critical, but maybe we could improve it at some point.

Closes #3706

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Also, it might be time to split out context pooling (and compiled models?) out into their own pages, they're big.

[ajcvickers]

Looks good. Given reason issues filed, maybe also add a note about not messing with the underlying ADO.NET state (DbConnection, transaction, etc.) or at least cleaning/resetting it appropriately.

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@ajcvickers good idea, done.