docs: add examples.org

This adds the basic framework of docs/examples.org, including the former
contents of demo.org in :lang emacs-lisp. elisp-demo has also been
reconfigured to search it instead.

Keep in mind that examples.org references a few things in as-of-yet
published documentation. This will be rectified soon.

modules/lang/emacs-lisp/autoload.el

@@ -369,6 +369,31 @@ library/userland functions"
 ;;
 ;;; Advice
 
+;;;###autoload
+(defun +emacs-lisp--add-doom-elisp-demos-a (fn symbol)
+  "Add Doom's own demos to `elisp-demos'.
+
+Intended as :around advice for `elisp-demos--search'."
+  (let ((org-inhibit-startup t)
+        enable-dir-local-variables
+        org-mode-hook)
+    (or (funcall fn symbol)
+        (with-file-contents! (doom-path doom-docs-dir "examples.org")
+          (save-excursion
+            (when (re-search-backward
+                   (format "^\\*+[ \t]+\\(?:TODO \\)?%s$"
+                           (regexp-quote (symbol-name symbol)))
+                   nil t)
+              (forward-line 1)
+              (let ((demos
+                     (string-trim
+                      (buffer-substring-no-properties
+                       (point) (if (re-search-forward "^\\*+ " nil t)
+                                   (line-beginning-position)
+                                 (point-max))))))
+                (unless (string-blank-p demos)
+                  demos))))))))
+
 ;;;###autoload (put 'map! 'indent-plists-as-data t)
 ;;;###autoload
 (defun +emacs-lisp--calculate-lisp-indent-a (&optional parse-start)