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/editor/file-templates/templates/org-mode/__doom-docs

@@ -1,9 +1,7 @@
 # -*- mode: snippet -*-
 # name: Doom documentation
 # --
-← [[doom-index:][Back to index]]                                ↖ [[doom-docs-source:faq.org][Source]]  ± [[doom-suggest-edit:][Suggest edits]]  ? [[doom-help:][Help]]
---------------------------------------------------------------------------------
-#+title:     ${1:Title}
-#+subtitle:  ${2:Subtitle}
+#+title:    ${1:Title}
+#+subtitle: ${2:Subtitle}
 
 $0