refactor!: replace all-the-icons with nerd-icons

BREAKING CHANGE: This commit replaces all-the-icons with nerd-fonts. Any
all-the-icons-* function calls or variable references in your private
config will break and should be replaced with their nerd-icons-*
equivalent. That said, Doom will continue to install all-the-icons for
a while, so feel free to load it if you don't want to fully commit to
the change yet.

This change is happening because nerd-icon has wider support for GUI and
TUI Emacs; has a larger, more consistent selection of symbols; plus unicode
coverage.

Fix: #7368
Close: #6675
Close: #7364

lisp/lib/docs.el

@@ -102,7 +102,7 @@
   "Create a header link according to SPEC."
   (let ((icon (and (plist-get spec :icon)
                    (funcall (or (plist-get spec :icon-function)
-                                #'all-the-icons-material)
+                                #'nerd-icons-material)
                             (plist-get spec :icon))))
         (label (pcase (plist-get spec :label)
                  ((and (pred functionp) lab)