Add example to version_ranges section with `git describe`

In the [version ranges](https://docs.conan.io/en/latest/mastering/version_ranges.html) doc page we can add an example using `git describe` as @Mark-Hatch-Bose suggests here (https://github.com/conan-io/conan/issues/4630#issuecomment-468781150). Maybe at the end of the page, like an example of application or real use case.