feat: remove gateway and federation modules

[codeflyer]

@jonnydgreen @mcollina a question related docs, examples and benchmarks.

Where do we want to keep them.

I was thinking to keep the docs for the gateway in mercurius.

Instead benchmark and examples will be in his own repo.

[jonnydgreen]

R.e. where to keep docs, very good question. I think at the very least, the Mercurius website (mercurius.dev) should be the entry point for developers. Now that things are split up into packages, I think there are several options for implementation, each with their own advantages and disadvantages

Potential solution	Notes
Provide links to each package docs from mercurius.dev	Scalable. Lose initial information (it's still there but you have to link out) about gateways - I think this is core info that should be retained in the website in some way
README file in each package repo but also have full docs about each package in core mercurius repo as you suggested	No loss in info, but hard to keep in sync - must be vigilant on this
Build a website that combines docs from the github pages site for each repo	Scalable but fair bit of initial work

WDYT?

[codeflyer]

I think the best solution in the long term is a repo with just the docs. Even if there is some work to do in the beginning is a required step to create a common place for the mercurius ecosystem. As a developer that's deciding to adopt mercurius as GQL server I consider it a plus to have access to a complete solution that solves the main issues. With the federation integrated this wasn't a big problem but with the separation I thinks is important to show that the product is unique even if is split in modules.

[marco-ippolito]

replaced by: https://github.com/mercurius-js/mercurius/pull/942 we can close this