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-readme

@@ -1,8 +1,6 @@
 # -*- mode: snippet -*-
 # name: Doom module readme                                             
 # --
-← [[doom-module-index:][Back to module index]]               ↙ [[doom-module-issues:::lang rust][Issues]]  ↖ [[doom-module-source:lang/rust][Source]]  ± [[doom-suggest-edit:][Suggest edits]]  ? [[doom-help-modules:][Help]]
---------------------------------------------------------------------------------
 #+title:     `(+file-templates-module-for-path)`
 #+subtitle:  <A one-line quip about this module to display in init.example.el>
 #+created:   `(format-time-string "%B %d, %Y")`