docs: added multiselect type docs

docs/syntax/types/multiselect.md

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+# Multiselect
+
+The `multiselect` type can be used to define variables that accept multiple options from a list of predefined options.
+
+## Basic
+
+Basic syntax for the `multiselect` type:
+
+```yaml
+variables:
+  - name: Colors
+    type: multiselect # Set the type to multiselect
+    options: # Define the options
+      - name: Red
+      - name: Green
+      - name: Blue
+    description: Favorite colors
+template: |-
+  Your favorite colors are {{ .Colors }}.
+```
+
+## Custom values
+
+You can use the `value` property to define custom values for the options:
+
+```yaml
+variables:
+  - name: Colors
+    type: multiselect
+    options:
+      - name: Red
+        value: "#ff0000" # Set the value to a hex color code
+      - name: Green
+        value: "#00ff00"
+      - name: Blue
+        value: "#0000ff"
+    description: Favorite colors
+template: |-
+  Your favorite colors are {{ .Colors }}.
+```
+
+## Iterate over results
+
+You can use the `range` function in the template to iterate over the results:
+
+```yaml
+variables:
+  - name: Colors
+    type: multiselect
+    options:
+      - name: Red
+      - name: Green
+      - name: Blue
+    description: Favorite colors
+template: |-
+  Your favorite colors are:
+  {{- range .Colors }}
+  - {{ . }}
+  {{- end }}
+```
+
+When every color is selected, the output will be:
+
+```
+Your favorite colors are:
+- Red
+- Green
+- Blue
+```