Update completions

Found https://github.com/FichteFoll/UnofficialDocs/pull/37 and https://github.com/FichteFoll/UnofficialDocs/pull/30 while checking our still open issues for things to include/merge and ended up revising the whole thing.

I intend to split the file into extensibility and reference again, where the "File Format" section would be moved to the reference completely and instead I would add some links to it from this introductory file. That way it can serve as "the knowledge base for completion interactions and usage" while guidance on how to create completions would be in the respective reference files.