docs: bring warning on shadowing context correctly from 0.6

[agraboso]

Brings the section titled "Warning: Shadowing Context Correctly" in the documentation for provide_context back from the 0.6 docs, updating use leptos::* to use leptos::prelude::* in the examples, and adding a use leptos::context::Provider in the appropriate example.

[gbj]

Yeah this may be the reason I didn't bring over this piece of the docs initially, as it inevitably brings in a leptos dependency, whereas reactive_graph is supposed to be a much more general-purpose and independent crate. Maybe this discussion should be moved somewhere else in Leptos docs or in the book. I don't have a great answer.

[agraboso]

That makes sense. Maybe the best place for it is chapter 8 of the book, in the section titled Option #2: Passing Signals through Context (with some modifications to adapt it to the narrative there).