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/completion/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:   :completion
 #+created: July 29, 2021
 #+since:   21.12.0

modules/completion/company/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :completion company
 #+subtitle: The ultimate code completion backend
 #+created:  February 19, 2017

modules/completion/helm/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :completion helm
 #+subtitle: A tank for hunting a house fly
 #+created:  February 20, 2017

modules/completion/ido/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :completion ido
 #+subtitle: A foil for other search engines
 #+created:  February 20, 2017

modules/completion/ivy/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :completion ivy
 #+subtitle: Yesterday's lightest search engine
 #+created:  February 20, 2017

modules/completion/vertico/README.org

@@ -1,3 +1,4 @@
+# -*- mode: doom-docs-org -*-
 #+title:    :completion vertico
 #+subtitle: Tomorrow's search engine
 #+created:  July 25, 2021