improved std::hash::Hash docs

Part of #29357.
* merged "Derivable" and "How can I implement `Hash`?" sections into one
  "Implementing `Hash`" section to aid coherency
* added an example for `#[derive(Hash)]`
* moved part about relation between `Hash` and `Eq` into a new "`Hash` and
  `Eq`" section; changed wording to be more consistent with `HashMap` and
  `HashSet`'s
* explicitly mentioned `#[derive(PartialEq, Eq, Hash)]` in the new section
* changed method summaries for consistency, adding links and examples

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