Documentation: remove use of "expandable" sections

Move the "Custom Job Types" expandable section to its own, normal
section.

The expandable sections in the docs aren't that clear, visually. It's
easy to miss that they can be expanded. This was the only use of this
feature.

web/project-website/content/usage/variables/_index.md

@@ -30,16 +30,6 @@ variables:
 Whenever a Worker gets a task that contains `{blender}`, that'll be replaced by
 the appropriate value for that worker.
 
-{{< expand "Custom Job Types" >}}
-This documentation section focuses on pre-existing variables, `blender` and
-`blenderArgs`. There is nothing special about these. Apart from being part of
-Flamenco's default configuration, that is. When you go the more advanced route
-of creating your own [custom job types][jobtypes] you're free to create your own
-set of variables to suit your needs.
-
-[jobtypes]: {{< ref "usage/job-types" >}}
-{{< /expand >}}
-
 ## Platform
 
 **The goal of the variables system is to cater for different platforms.**
@@ -64,3 +54,13 @@ might be different from where users go to pick them up.
 - `users`: values are used when submitting jobs from Blender and showing them in
   the web interface.
 - `workers`: values that are used when sending tasks to workers.
+
+## Custom Job Types
+
+This documentation section focuses on pre-existing variables, `blender` and
+`blenderArgs`. There is nothing special about these. Apart from being part of
+Flamenco's default configuration, that is. When you go the more advanced route
+of creating your own [custom job types][jobtypes] you're free to create your own
+set of variables to suit your needs.
+
+[jobtypes]: {{< ref "usage/job-types" >}}