refactor documentation

docs/config-variables.md

@@ -0,0 +1,36 @@
+# Variables entry
+
+The **variables** entry (optional) contains a variables mapping (See [variables](config-file.md#variables)).
+
+```yaml
+variables:
+  <variable-name>: <variable-content>
+```
+
+Variables defined in the `variables` entry are made available within the config file.
+
+Config variables are recursively evaluated, which means that
+a config like the below:
+```yaml
+variables:
+  var1: "var1"
+  var2: "{{@@ var1 @@}} var2"
+  var3: "{{@@ var2 @@}} var3"
+  var4: "{{@@ dvar4 @@}}"
+dynvariables:
+  dvar1: "echo dvar1"
+  dvar2: "{{@@ dvar1 @@}} dvar2"
+  dvar3: "{{@@ dvar2 @@}} dvar3"
+  dvar4: "echo {{@@ var3 @@}}"
+```
+
+will result in the following available variables:
+
+* var1: `var1`
+* var2: `var1 var2`
+* var3: `var1 var2 var3`
+* var4: `echo var1 var2 var3`
+* dvar1: `dvar1`
+* dvar2: `dvar1 dvar2`
+* dvar3: `dvar1 dvar2 dvar3`
+* dvar4: `var1 var2 var3`