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/term/README.org

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

modules/term/eshell/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :term eshell
 #+subtitle: The elisp shell that works everywhere
 #+created:  February 20, 2017

modules/term/shell/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :term shell
 #+subtitle: A REPL for your shell
 #+created:  August 01, 2021

modules/term/term/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :term term
 #+subtitle: It's terminal
 #+created:  August 01, 2021

modules/term/vterm/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :term vterm
 #+subtitle: As good as terminal emulation gets in Emacs
 #+created:  January 16, 2019