[docs] Expand on controllers & directive doc

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Suggested by @bennypowers

We have a section in the docs describing combinations of controllers and directives: https://lit.dev/docs/composition/controllers/#controllers-and-directives

It would be good to add a bit more details there, maybe show some code and/or link to examples.

Directive controllers -> Add more concrete code of how to add controller to host from directive's update? Ex. `@lit-labs/motion`'s `animate()` directive.

Controllers owning directive -> We have some incomplete code example in the doc. Could add more to that. Another example is final form integration controller exposing directive for binding https://lit.dev/playground/#gist=08b866474027f36eaa0d1863a61c7065

Start with the Controllers and Directives section at the linked lit.dev documentation page. Review the @lit-labs/motion animate() directive and the final form integration controller playground example for concrete patterns. Done means the section has clearer code examples or links explaining both directive controllers and controllers that own directives.

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