make navigation and header sticky

apps/app/src/components/Layout/PageViewLayout.tsx

@@ -19,7 +19,7 @@ export const PageViewLayout = (props: Props): JSX.Element => {
       <div id="main" className={`main page-view-layout ${styles['page-view-layout']}`}>
         <div id="content-main" className="content-main container-lg grw-container-convertible">
           { headerContents != null && (
-            <header className="sticky-top">
+            <header className="sticky-top mb-4">
               {headerContents}
             </header>
           ) }

apps/app/src/pages/[[...path]].page.tsx

@@ -325,9 +325,9 @@ const Page: NextPageWithLayout<Props> = (props: Props) => {
         <title>{title}</title>
       </Head>
       <div className={`dynamic-layout-root ${growiLayoutFluidClass} justify-content-between`}>
-        <header className="py-0 position-relative">
+        <nav className="sticky-top">
           <GrowiContextualSubNavigation isLinkSharingDisabled={props.disableLinkSharing} />
-        </header>
+        </nav>
 
         <DisplaySwitcher
           pageView={(