Added "relative_to_docs" value to absolute_links enum (#7812)

* Add `"relative_to_docs"` value to `absolute_links` enum

* refactor to use `oneOf`

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Co-authored-by: Jake Beazley <jake.beazley@johnlewis.co.uk>

Author: bzly

docs/schema/validation.json

@@ -43,7 +43,10 @@
     },
     "absolute_links": {
       "title": "warning level when absolute links are used",
-      "$ref": "#/$defs/warning_levels"
+      "oneOf": [
+        { "$ref": "#/$defs/warning_levels" },
+        { "enum": ["relative_to_docs"] }
+      ]
     },
     "unrecognized_links": {
       "title": "warning level when a relative link cannot be resolved to a document",