revert: fix(docs): set mode in file-local vars

emacs-straight/org-mode@e22b4eb7aab9 introduced a breaking change that
made it impossible to rely on .dir-locals.el to change org-mode buffers
in $EMACSDIR to doom-docs-org-mode (without infinite recursion), so we
used file-local variables in 7e400ab.

emacs-straight/org-mode@215de6176b22 reverted that change, so we can use
.dir-locals.el again, and this is my preference, since it requires less
boilerplate across our hundreds of org files.

Ref: emacs-straight/org-mode@215de6176b22
Ref: emacs-straight/org-mode@e22b4eb7aab9
Revert: 7e400ab

Author: hlissner

.dir-locals.el

@@ -1,2 +1,3 @@
 ((nil (git-commit-major-mode . git-commit-elisp-text-mode)
-      (fill-column . 80)))
+      (fill-column . 80))
+ (org-mode (mode . doom-docs-org)))