chore: document standard code var syntax

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

content/contribute/components/code-blocks.md

@@ -8,6 +8,18 @@ Rouge provides lots of different code block "hints". If you leave off the hint,
 it tries to guess and sometimes gets it wrong. These are just a few hints that
 we use often.
 
+## Variables
+
+If your example contains a placeholder value that's subject to change,
+use the format `<[A-Z_]+>` for the placeholder value: `<MY_VARIABLE>`
+
+```none
+export name=<MY_NAME>
+```
+
+This syntax is reserved for variable names, and will cause the variable to
+be rendered in a special color and font style.
+
 ## Bash
 
 Use the `bash` language code block when you want to show a Bash script:
@@ -157,4 +169,4 @@ authorizedkeys:
     - AUTHORIZED_KEYS=ssh-rsa AAAAB3Nsomelongsshkeystringhereu9UzQbVKy9o00NqXa5jkmZ9Yd0BJBjFmb3WwUR8sJWZVTPFL
   volumes:
     /root:/user:rw
-```
+```