[apidoc] qoc indicator: Add the notion of 'index'

schemas/Indicator.yaml

@@ -16,6 +16,14 @@ properties:
     description: Short, in dutch, *HTML* (but rarely includes formatting; at most, sup/superscript).
     type: string
     example: Prevalentie Diabetes
+  index:
+    description: |-
+      *OPTIONAL* Indicators usually are based on documentation that tends to take the form of an
+      ordered list (like html `<ol>`), and it is convenient for indicators to copy this numbering scheme.
+      For most UI applications, simply prefix this to the title, e.g. render as `1. Title Here`.
+      If missing, do not show an index in the title.
+    type: number
+    example: 1
   xMin:
     description: Numeric value of the far left of the bar.
     type: number