flag page-specific editor overrides as deprecated

Since Confluence Cloud is deprecating its legacy editor, we will start
preparing for only using the `v2` editor on Confluence Cloud. To help
start with the transition, flagging the page-specific editor override
to warn users early of upcoming changes required.

Signed-off-by: James Knight <git@jdknight.me>

Author: jdknight

sphinxcontrib/confluencebuilder/builder.py

@@ -954,6 +954,15 @@ def _extract_metadata(self, docname, doctree):
                 else:
                     metadata[k] = v
 
+                if k == 'editor':
+                    msg = 'page-specific editor overrides are deprecated'
+                    if node.source:
+                        lpf = f'#{node.line}' if node.line else ''
+                        self.warn(f'{msg}: {node.source}{lpf}',
+                            subtype='deprecated')
+                    else:
+                        self.warn(f'{msg}: {docname}', subtype='deprecated')
+
             node.parent.remove(node)
 
     def _find_title_element(self, doctree):