Add examples for scenario reuse

It seems like with `scenarios`, people want to put different scenarios in different files and execute them separately, but also execute all of them at once. The requisite k6 issue for making this use case better is https://github.com/loadimpact/k6/issues/1342, but even now, people can achieve a lot by just having a `main` file that imports and re-exports individual scenarios. Recent examples where this has popped up:
- https://community.k6.io/t/run-two-several-scenario-files-in-parallel-or-serially-using-single-file/1584
- https://github.com/k6io/k6/issues/1918

Start by locating the existing scenarios documentation and examples in k6-docs, then review k6 issue #1342 and the cited use cases. Add examples showing a main file importing and re-exporting individual scenarios for separate or combined execution; done when both workflows are explained clearly.

Written by the indexing model from the issue text.