Allow to set comments.hypothesis.showHighlights = "whenSidebarOpen" (#4620)

cf. https://h.readthedocs.io/projects/client/en/latest/publishers/config.html#cmdoption-arg-showHighlights

Author: salim-b

src/resources/schema/definitions.yml

@@ -283,9 +283,9 @@
                     showHighlights:
                       anyOf:
                         - boolean
-                        - enum: ["always", "never"]
+                        - enum: ["always", "whenSidebarOpen", "never"]
                       default: "always"
-                      description: Controls whether the in-document highlights are shown by default (`always` or `never`)
+                      description: Controls whether the in-document highlights are shown by default (`always`, `whenSidebarOpen` or `never`)
                     theme:
                       enum: ["classic", "clean"]
                       default: classic