Remove attributes.derive.proc-macro

This moves the text into the intro. I think it's fine to have this be
more of a high-level introduction. The details and rules of derive
proc-macros are covered in the proc-macro chapter, so there doesn't need
to be an explicit rule here.

Author: ehuss

src/attributes/derive.md

@@ -2,7 +2,7 @@ r[attributes.derive]
 # Derive
 
 r[attributes.derive.intro]
-The *`derive` attribute* allows new [items] to be automatically generated for data structures.
+The *`derive` [attribute][attributes]* allows new [items] to be automatically generated for data structures. You can implement custom `derive` macros through [procedural macros].
 
 > [!EXAMPLE]
 > The following example will create an [`impl` item] for the [`PartialEq`] and [`Clone`] traits for `Foo`, and the type parameter `T` will be given the `PartialEq` or `Clone` constraints for the appropriate `impl`:
@@ -30,8 +30,6 @@ r[attributes.derive.syntax]
 It uses the [MetaListPaths] syntax to specify a list of traits to implement or paths to [derive macros] to process.
 
 
-r[attributes.derive.proc-macro]
-You can implement `derive` for your own traits through [procedural macros].
 
 r[attributes.derive.automatically_derived]
 ## The `automatically_derived` attribute