Help mode documentation for ternary operator

[c42f]

Fixes #35636

Here I've stolen the single ? as an alias for displaying the introduction text, as the intro text already has multiple other ways to show it (including simply pressing Return while in help mode).

I've also tried out something unusual which is linking to the manual documentation which describes this syntax in more detail. (Obviously we should have a better way to create such links without direct URLs, but for now I think this is the only way to do it?)

[c42f]

Thanks @fredrikekre for the review, I think I've figured out how to incorporate those suggestions without breaking things. Any further thoughts?

[fredrikekre]

Would be good to include it in the manual too, maybe in the same section as if in https://github.com/JuliaLang/julia/blob/master/doc/src/base/base.md

[c42f]

Would be good to include it in the manual too

Done + rebased+squashed.

[ViralBShah]

@c42f Please merge if ready to go.