Replace mkdocs-simple-hooks with built-in hooks so we don't need to
pacakge the plugin.
https://www.mkdocs.org/user-guide/configuration/#hooks
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -56,8 +56,5 @@ plugins:
     - build/*
     - examples/*
     - requirements.txt
-- mkdocs-simple-hooks:
-    hooks:
-      on_pre_build: 'docs.plugins:on_pre_build'
-      on_files: 'docs.plugins:on_files'
-      on_page_markdown: 'docs.plugins:on_page_markdown'
+hooks:
+  - docs/plugins.py