docs: properly document prometheus protocols

[ywwg]

name encoding schemes are only really documented in code. we should have a reference.

• new Protocol doc under Concepts
• update https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md

[ywwg]

changed the name of the bug to indicate its wider scope

[beorn7]

Maybe the "Specifications" section would be more appropriate than "Concepts"? The former is where the remote-write protocols are formally specified.

(In different news, I think we should reorganize the sections. Among others, I think it would make more sense to merge concepts and specifications, cf. #2547 .)

[beorn7]

In other different news, this issue might better live in prometheus/docs.