refactor(docs): replace nav menus with virtual one

Now that the header is dynamically generated (when doom-docs-mode is
active), a literal header is unnecessary.

modules/completion/company/README.org

@@ -1,5 +1,3 @@
-← [[doom-module-index:][Back to module index]]               ! [[doom-module-issues:::completion company][Issues]]  ↖ [[doom-repo:tree/develop/modules/completion/company/][Github]]  ± [[doom-suggest-edit:][Suggest edits]]  ? [[doom-help-modules:][Help]]
---------------------------------------------------------------------------------
 #+title:    :completion company
 #+subtitle: The ultimate code completion backend
 #+created:  February 19, 2017
@@ -15,6 +13,7 @@ https://assets.doomemacs.org/completion/company/overlay.png
 - [[doom-user:][@hlissner]]
 
 [[doom-contrib-maintainer:][Become a maintainer?]]
+
 ** Module flags
 - +childframe ::
   Display completion candidates in a [[https://www.gnu.org/software/emacs/manual/html_node/elisp/Child-Frames.html][child frame]] rather than an overlay or