Little bit more documentation for the noble @easy_rule (#2695)

* Add a really simple example to @easy_rule

* Another easy rule example and xref

---------

Co-authored-by: Katharine Hyatt <[email protected]>

Author: kshyatt

docs/src/index.md

@@ -307,7 +307,7 @@ Enzyme also supports a second way to mark things inactive, where the marker is "
 EnzymeRules.inactive_noinl(::typeof(det), ::UnitaryMatrix) = true
 ```
 
-### Easy Rules
+### [Easy Rules](@id man-easy-rule)
 
 The recommended way for writing rules for most use cases is through the [`EnzymeRules.@easy_rule`](@ref) macro. This macro enables users to write derivatives for any functions which only read from their arguments (e.g. do not overwrite memory), and has numbers, matricies of numbers, or tuples thereof as arguments/result types. 
 

lib/EnzymeCore/src/easyrules.jl

@@ -719,6 +719,31 @@ If a specific argument has no partial derivative, then all corresponding argumen
              ...)
 ```
 
+# Examples
+
+Let's write an `@easy_rule` for a simple trigonometric function. Enzyme already has rules for `sin` and `cos`, but for the sake of illustration we can define a new pass-through function to oen of them to demostrate the `@easy_rule` interface.
+
+```julia
+mycos(x) = cos(x)
+
+# forward-mode rule for the new function
+EnzymeRules.@easy_rule(
+    mycos(x::AbstractFloat),
+    @setup(),
+    (-sin(x),)
+)
+```
+
+Then this rule can be tested by running `Enzyme.autodiff` and comparing with the result for the regular `cos` function:
+
+```julia
+myderiv   = autodiff(Forward, mycos, Duplicated(2.0f0, 1.2f0))[1] # -1.091157f0
+truederiv = autodiff(Forward, cos, Duplicated(2.0f0, 1.2f0))[1] # -1.091157f0
+@assert myderiv = truederiv
+```
+
+For more information about easy rules, see the [manual](@ref man-easy-rule).
+
 """
 macro easy_rule(call, maybe_setup, partials...)
     call, setup_stmts, inputs, input_names, normal_inputs, partials = _normalize_scalarrules_macro_input(
@@ -735,4 +760,4 @@ macro easy_rule(call, maybe_setup, partials...)
         $(frule_expr)
         $(rrule_expr)
     end
-end
+end