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/app/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:   :app
 #+created: February 19, 2017
 #+since:   21.12.0

modules/app/calendar/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :app calendar
 #+subtitle: Watch your missed deadlines in real time
 #+created:  January 13, 2018

modules/app/emms/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :app emms
 #+subtitle: A media player for music no one's heard of
 #+created:  March 06, 2021

modules/app/everywhere/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :app everywhere
 #+subtitle: *leave* Emacs!? You must be joking
 #+created:  February 07, 2021

modules/app/irc/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :app irc
 #+subtitle: How neckbeards socialize
 #+created:  June 11, 2017

modules/app/rss/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :app rss
 #+subtitle: An RSS reader that Google can't shut down
 #+created:  May 12, 2020

modules/app/twitter/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :app twitter
 #+subtitle: Be superficial in plain text
 #+created:  October 11, 2019