fix(templates): render the documented function value in shell rc files

The template read `function.body`, while README and defaults/main.yaml document the key as `functions[].value`. Every
documented example therefore rendered an empty function body, and with a strict undefined policy the template failed
outright.

The documentation is the contract for this role, so the template follows it instead of the other way around.

Co-authored-by: Copilot <copilot@github.com>

templates/shell_rc_file.j2

@@ -5,7 +5,7 @@
 {% if shell_rc_file.functions is defined %}
 {% for function in shell_rc_file.functions %}
 function {{ function.name }} {
-{{ function.body | indent(2, True) }}
+{{ function.value | indent(2, True) }}
 }
 
 {% endfor %}