Format all instructions.append.md to start with an H1 (required but ignored) and H2 (#1633)

* Format all `instructions.append.md` to start with an H1 (required but ignored) and H2

* Update the header to "Track specific instructions"

* Reuse the existing Elixir-specific header

Author: IsaacG

exercises/practice/complex-numbers/.docs/instructions.append.md

@@ -1,5 +1,7 @@
 # Instructions append
 
+## Elixir-specific changes
+
 In this exercise, complex numbers are represented as a tuple-pair containing the real and imaginary parts.
 
 For example, the real number `1` is `{1, 0}`, the imaginary number `i` is `{0, 1}` and the complex number `4+3i` is `{4, 3}`.

exercises/practice/custom-set/.docs/instructions.append.md

@@ -1,3 +1,5 @@
 # Instructions append
 
+## Elixir-specific changes
+
 For this exercise please refrain from using the `MapSet` API.

exercises/practice/resistor-color/.docs/instructions.append.md

@@ -1,3 +1,5 @@
 # Instructions append
 
+## Elixir-specific changes
+
 For this exercise, returning the list of colors is not required.

exercises/practice/square-root/.docs/instructions.append.md

@@ -1,3 +1,5 @@
 # Instructions append
 
+## Elixir-specific changes
+
 Make sure you implement an algorithm that doesn't rely on built-in functions such as `:math.sqrt/1` or `Float.pow/2`.