Add instructions for partners to enable module by default, and in jumpstart

Currently our [partner docs](docs/partners/managing-modules.md) describe how to hide, force-enable or force-disable a module.

It's also desirable to enable a module by default (while still allowing it to be disabled), or to enable a module via Jumpstart (aka "recommended modules"). We should add descriptions (and, if necessary, appropriate filters) to allow this.