Terminology: gateway v.s. distributed executor

[SimonSapin]

The term "gateway" is use a few times across this repository. For example:

composite-schemas-spec/spec/Section 1 -- Overview.md

Lines 43 to 45 in aaf454b

	To enable greater interoperability between different implementations of tooling
	and gateways, this specification focuses on two core components: schema
	composition and distributed execution.

Based on the existence of @apollo/gateway I can guess it is used interchangeably with "distributed (GraphQL) executor", which has a proper definition in the Overview. I suggest using "execution" or "distributed executor" everywhere. (No need to repeat "GraphQL" every time, this being a document of the GraphQL WG.)

[benjie]

Agree that we should standardize on terminology and use the same terms everywhere - we can use spec-md's definitions feature to link to the definition of the word.

This may be a duplicate of #17 where the correct term to use is under discussion?

[SimonSapin]

Indeed, closing as duplicate of #17