Add an explicit "How can I implement `PartialEq`"? doc section
Add an explicit "How can I implement `PartialEq`"? doc section

Including an example of a custom implementation. I put this expanded
section after the `Derivable` section to encourage use of that first.

GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust