Add document page to assists

[FMorschel]

What information needs to be added?

A list of all the asists available and where on the code do they trigger. What change will that make, which could be copied from the current tests.

I believe that would help people be more familiar with them.

Where should this new content appear?

Under the docs session.

I would like to fix this problem.

• I will try and fix this problem on dart.dev.

[bwilkerson]

If we do this we should also include documentation of the refactorings. (That might be implied by the original request, but we make a distinction in the implementation between assists and refactorings, so I wanted it to be stated explicitly.) Documenting the fixes is less clear given that they're generally easy to find (though the change they make might not be obvious in all cases).

[DanTup]

Slightly related - VS Code allows us to provide some text/a command that can be shown at the bottom of the refactor list:

microsoft/vscode#86788

It's not per-action documentation, we just get to say "If the list is for CodeAction.Refactor, include this documentation action" (that calls a command). If the refactorings were documented on the website, we could add this and link to them so that (VS Code) users can easily find them.