aio.md: multiple improvements

Pull Request: carpentries/styles#406

* change all-in-one file to have the same 'depth' as episode files

* move the core script of aio.md into _includes/aio-script.md

* include comment to inform maintainers to not edit the file

* move aio.md to the root of styles repository

bin/boilerplate/aio.md → _includes/aio-script.md

@@ -1,7 +1,8 @@
----
----
-
-{% include base_path.html %}
+{% comment %}
+As a maintainer, you don't need to edit this file.
+If you notice that something doesn't work, please 
+open an issue: https://github.com/carpentries/styles/issues/new
+{% endcomment %}
 
 <script>
   window.onload = function() {

aio.md

@@ -0,0 +1,13 @@
+---
+permalink: /aio/index.html
+---
+
+{% comment %}
+As a maintainer, you don't need to edit this file.
+If you notice that something doesn't work, please 
+open an issue: https://github.com/carpentries/styles/issues/new
+{% endcomment %}
+
+{% include base_path.html %}
+
+{% include aio-script.md %}