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/lang/markdown/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :lang markdown
 #+subtitle: Write docs for people to ignore
 #+created:  February 19, 2017