fix(docs): set mode in file-local vars

Due to emacs-straight/org-mode@e22b4eb, a replacement major mode (for
org-mode) can no longer be specified from .dir-locals.el, as it spirals
into infinite recursion. Therefore, a mode: line is needed in all Doom
docs.

Ref: emacs-straight/org-mode@e22b4eb7aa

modules/emacs/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:   :emacs
 #+created: August 01, 2021
 #+since:   21.12.0

modules/emacs/dired/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :emacs dired
 #+subtitle: Making dired pretty [functional]
 #+created:  February 20, 2017

modules/emacs/electric/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :emacs electric
 #+subtitle: Shocking keyword-based electric-indent
 #+created:  February 21, 2017

modules/emacs/ibuffer/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :emacs ibuffer
 #+subtitle: Edit me like one of your French buffers
 #+created:  October 20, 2019

modules/emacs/undo/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :emacs undo
 #+subtitle: Persistent, smarter undo for your inevitable mistakes
 #+created:  April 14, 2020

modules/emacs/vc/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :emacs vc
 #+subtitle: Be the difference you want to see in the fringe
 #+created:  February 20, 2017