Add _integrate_forces boilerplate

Currently the documentation for the [`_integrate_forces`](https://docs.godotengine.org/en/stable/classes/class_rigidbody3d.html#class-rigidbody3d-private-method-integrate-forces) function explains what it's for but doesn't provide examples on how it might be used, either with or without the [`_custom_integrator`](https://docs.godotengine.org/en/stable/classes/class_rigidbody3d.html#class-rigidbody3d-property-custom-integrator) property activated.

Often I have found myself wanting to remove/change just one part of how forces are integrated, but not wanting to rewrite an entire integrator alongside that change. It would be nice to have a boilerplate/standard integrator example in the docs, showing what the default force integration looks like, so that devs can read, understand, and modify the default behaviour, without having to go through the source code.