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/email/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:   :email
 #+created: October 14, 2021
 #+since:   21.12.0

modules/email/mu4e/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :email mu4e
 #+subtitle: The great filter Hanson hadn't anticipated
 #+created:  April 08, 2017

modules/email/notmuch/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :email notmuch
 #+subtitle: Closest Emacs will ever be to multi-threaded
 #+created:  July 31, 2018

modules/email/wanderlust/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :email wanderlust
 #+subtitle: To boldly go where no mail has gone before
 #+created:  May 05, 2019