Rework the Concepts chapter

[IchHabeHunger54]

Next up in the series of major rewrites: the Concepts chapter! This PR:

• Rewrites the Registries page. Among other things, the page now contains the contents of the registry rework, as well as information on datapack registries.
• Rewrites the Events page. It now contains the changes since the split from Forge (for example the ICancellable interface), and is just better to read in general.
• Rewrites and significantly simplifies the Sides page.
• Removes the old mini-Resources article and replaces it with a more detailed article on Resource Locations and Resource Keys in the Miscellaneous section.
• Merges the Lifecycle article into the Events page.
• Merges the Datapack Registry Datagen article into the Registries page.
• Adds information about DeferredRegister.Blocks and DeferredRegister.Items to the blocks and items articles, respectively.
• Changes a lot of links due to the restructuring.

Please proofread the new articles before approving. This goes especially for the registries part, since I'm not certain I got everything correct there.

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