chore: Update to mkdocs-simple v2.1.2 (#23)

Note the upstream refactor from mkdocs-simple v1 -> v2 was fairly large, so
  this is a rather larger update than most. And in fact one feature of
  mkdocs-semiliterate (universal markdown extraction) was incorporated into
  mkdocs-simple, so the initialization of this plugin is noticeably simplified
  as a result.

  Also updates mkdocs to 1.4.

Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: #23

tests/fixtures/theme-modification/mkdocs.yml

@@ -5,7 +5,8 @@ theme:
   custom_dir: doc_theme/
 plugins:
 - semiliterate:
-    ignore_folders: [refsite, snippet]
+    ignore_folders: [refsite]
+    ignore_hidden: false
     merge_docs_dir: false
     include_extensions: []
     semiliterate: