Improve middleware docs

[burakcan]

Hello.
I've started using Inngest and reading through the documents, I've noticed that it doesn't explain "why" we need middlewares in certain situations.
For example I've tried using prisma with middleware or directly in a function, they both work. But I couldn't find what is the functional difference between the two. Adding some explanation to the relevant section of the documentation would be nice,

[linear]

INN-2856 Improve middleware docs