feat(developer): add IPreloadableNotifier to upgrade guide

[st3iny]

☑️ Resolves

• For Provide INotifier::preloadMany() to allow implementations to preload and cache data for many notifications at once server#54231
• PR at feat(notifications): provide method to preload many notifications at once server#54232

🖼️ Screenshots

[ChristophWurst]

👍 on listing this in the upgrade guide so app devs can check their notifiers and modify them if needed

But this should also have a dedicated page, regardless of the 32 changes, with an example to demonstrate the purpose of the preload

[nickvergessen]

That would then be https://github.com/nextcloud/notifications/blob/master/docs/notification-workflow.md

Or we move all this over at some point.

Since I hold a lightning Talk about this, I can take care of it when preparing that.

[ChristophWurst]

Personally I would prefer to have this in the dev manual where we also cover the rest of OCP, so 👍 on moving it over :)

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