Show an example of solver rejection in docs

> The documentation doesn't have any example of solver rejection?

Searching for "rejecting" in `*.rst` gets no hits.

_Originally posted by @philderbeast in https://github.com/haskell/cabal/issues/9824#issuecomment-2010092134_ in response to a question posed by @Kleidukos.

I remember when first trying out cabal, seeing the solver rejection message and not being able to come up with a solvable set of versions. It would be nice to have some walk-throughs on how to deal with these with examples.

The issue points to documentation in *.rst; start by searching existing solver and dependency-resolution sections, including the absence of hits for "rejecting." Add a worked example or walkthrough for solver rejection, and verify that the documentation clearly explains how to handle the scenario.

Written by the indexing model from the issue text.