[docs] Reactive Properties page never actually mentions how to pass properties to child components

Nobody has claimed this yet.

### Feature Request
Add a pointer to the [Property Expressions](https://lit.dev/docs/templates/expressions/#property-expressions) section from the [Reactive Properties page](https://lit.dev/docs/components/properties/).

### Context
The [Reactive Properties page](https://lit.dev/docs/components/properties/) does a good job of explaining properties and state, but somehow never mentions explicitly how a Lit developer should pass a property to a child component.

It seems that information is found at the [Expressions page](https://lit.dev/docs/templates/expressions/) which is found in the "Templates" sub-section of the LHS nav, but this is not obvious (why should I know it's there?), and **searching for "properties" in the UI search element doesn't even return this page as a result**.

Start with the Reactive Properties page and the linked Property Expressions section to compare where property passing is currently explained. Add a pointer from the Reactive Properties page to Property Expressions, then verify that the page explicitly directs readers to the relevant guidance.

Written by the indexing model from the issue text.