docs: Clarification Middleware vs Plugins

[MentalGear]

Would it be possible to add to the FAQ how this middleware differs from Apollo Server new plugin system?
There seems to be much overlap, hence it would be helpful for visitors to quickly get an idea where the differences lie.

[maticzav]

@MentalGear, thank you for pointing this out. I agree that there's a large overlap, but I am not that familiar with Apollo Server. We hoped that they would, at some point, make this a part of Apollo Server, but they didn't want to. From what I can tell from the discussions, Apollo Server isn't as granular when it comes to resolvers but has much more extensive coverage of the query execution process.

I'd be delighted to merge a PR explaining the difference, even if it's only a sentence or two.