[docs] move NEWS to the documentation

[odow]

The Python ecosystem do this. I think it works quite well: http://xarray.pydata.org/en/latest/whats-new.html

It's doubly important for this release of MOI because there are such a large number of changes.

[odow]

I think this is useful: https://jump.dev/MathOptInterface.jl/previews/PR1565/release_notes/

[blegat]

Why not keeping NEWS.md where it is ? So that it can be accessed directly in addition to being in the docs ? An advantage of having it in the docs is that we can use Documenter features such as @ref. In that case, we can move it in docs/src but then we would need a clear link to if from the README

[odow]

Why not keeping NEWS.md where it is ? So that it can be accessed directly in addition to being in the docs ?

You can still link directly: https://github.com/jump-dev/MathOptInterface.jl/blob/od/release_notess/docs/src/release_notes.md

How often to people actually check the NEWS.md file? Adding it to the docs makes it much more visible. We could add a placeholder NEWS.md that just says "check the docs"?

having it in the docs is that we can use Documenter features such as @ref

I thought about this, but then when we remove things like SingleVariable you just break all the previous links. We could get better at writing detailed release notes with links to the PRs through.