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/input/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:   :input
 #+created: May 13, 2019
 #+since:   2.0.9

modules/input/chinese/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :input chinese
 #+subtitle: Spend your 3 hours a week in Emacs
 #+created:  May 13, 2019

modules/input/japanese/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :input japanese
 #+subtitle: Ah, a man of culture
 #+created:  May 13, 2019

modules/input/layout/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :input layout
 #+subtitle: auie,ctsrnm is the superior home row
 #+created:  Jun 29, 2020