feat: add renderTopRight prop & remove GH corner from core (#3539)

* feat: add `renderTopRight` prop & remove GH corner from core

* reuse `--space-factor` var

* update readme & changelog

src/components/App.tsx

@@ -452,6 +452,7 @@ class App extends React.Component<AppProps, AppState> {
     const {
       onCollabButtonClick,
       onExportToBackend,
+      renderTopRight,
       renderFooter,
       renderCustomStats,
     } = this.props;
@@ -492,6 +493,7 @@ class App extends React.Component<AppProps, AppState> {
               langCode={getLanguage().code}
               isCollaborating={this.props.isCollaborating || false}
               onExportToBackend={onExportToBackend}
+              renderTopRight={renderTopRight}
               renderCustomFooter={renderFooter}
               viewModeEnabled={viewModeEnabled}
               showExitZenModeBtn={