Crosslink FAQ to Changing the Primal

Author: oxinabox

docs/src/FAQ.md

@@ -30,6 +30,8 @@ There are three reasons the rules also calculate the `f(x)`.
 2. For many `rrule`s the output value is used in the definition of the pullback. For example `tan`, `sigmoid` etc.
 3. For some `frule`s there exists a single, non-separable operation that will compute both derivative and primal result. For example many of the methods for [differential equation sensitivity analysis](https://docs.juliadiffeq.org/stable/analysis/sensitivity/#sensitivity-1).
 
+For more information and examples see the [design notes on changing the primal](@ref change_primal).
+
 ## Where are the derivatives for keyword arguments?
 _pullbacks_ do not return a sensitivity for keyword arguments;
 similarly _pushfowards_ do not accept a perturbation for keyword arguments.

docs/src/design/changing_the_primal.md

@@ -1,4 +1,4 @@
-# Design Notes: Why can you change the primal computation?
+# [Design Notes: Why can you change the primal computation?](@id change_primal)
 
 These design notes are to help you understand ChainRules.jl's [`rrule`](@ref) function.
 It explains why we have a `rrule` function that returns both the primal result (i.e. the output for the forward pass) and the pullback as a closure.