Remove Portal from docs and index.ts until it is ready to be shipped (see #1025).

Author: T-Hugs

docs/src/@primer/gatsby-theme-doctocat/nav.yml

@@ -80,8 +80,8 @@
       url: /PointerBox
     - title: Popover
       url: /Popover
-    - title: Portal
-      url: /Portal
+    # - title: Portal
+    #   url: /Portal
     - title: Position
       url: /Position
     - title: ProgressBar

src/index.ts

@@ -50,7 +50,7 @@ export {default as Pagehead} from './Pagehead'
 export {default as Pagination} from './Pagination'
 export {default as PointerBox} from './PointerBox'
 export {default as Popover} from './Popover'
-export {default as Portal, PortalProps, registerPortalRoot} from './Portal'
+// export {default as Portal, PortalProps, registerPortalRoot} from './Portal'
 export {default as ProgressBar} from './ProgressBar'
 export {default as SelectMenu} from './SelectMenu'
 export {default as SideNav} from './SideNav'