Rename doomdirs/example and doom-example

Move `doomdirs/example/` to just `doomdir/`, and rename the package
using it from `doom-example` to `doom-emacs`.

I do not expect to need multiple doomdirs checked in again, and I can
unclutter the packages provided by the flake a bit now that most of them
are checks.

doomdir/packages.el

@@ -0,0 +1,20 @@
+;; Copyright 2024 Google LLC
+;;
+;; Licensed under the Apache License, Version 2.0 (the "License");
+;; you may not use this file except in compliance with the License.
+;; You may obtain a copy of the License at
+;;
+;;     http://www.apache.org/licenses/LICENSE-2.0
+;;
+;; Unless required by applicable law or agreed to in writing, software
+;; distributed under the License is distributed on an "AS IS" BASIS,
+;; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+;; See the License for the specific language governing permissions and
+;; limitations under the License.
+
+;; -*- no-byte-compile: t; -*-
+;;; $DOOMDIR/packages.el
+
+(package! evil-escape :disable t)
+
+(package! dotenv-mode)