Expand docs on `Deref` and `DerefMut`

Part of #29365.
* Expanded the explanaition sections, adapting some parts from the book,
  the reference, as well as the API guidelines. As such, the docs now
  explicitly state that `Deref` and `DerefMut` should only be implemented
  for smart pointers and that they should not fail. Additionally, there
  is now a short primer on `Deref` coercion.
* Added links to `DerefMut` from `Deref` and vice versa
* Added links to relevant reference sections
* Removed "stuttering" in summary sentences
* Changed summary sentences of `Deref::deref` and `Deref::deref_mut` to
  be in 3rd person singular
* Removed explicit uses of `fn main()` in the examples

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