No bzlmod example

The setup guide only explains how to use rules_foreign_cc with WORKSPACE, not with bzlmod.

https://bazelbuild.github.io/rules_foreign_cc/main/index.html#setup

Start with the setup guide at the linked documentation page and inspect how its WORKSPACE example is presented. Add a corresponding bzlmod example for rules_foreign_cc, keeping the setup guidance clear for users of both approaches. Done means the published setup guide documents bzlmod usage alongside WORKSPACE.

Written by the indexing model from the issue text.