feat: Allow semiliterate to adjust the theme

The mechanism here is to shadow a specified `custom.theme_dir` in the
  generated docs directory, and then ignore generated files therein (since they
  will be being used by the theme to render the pages, and are not actually
  part of the documentation).

tests/fixtures/theme-modification/doc_theme/.base.generator

@@ -0,0 +1,4 @@
+{! "\syspath mkdocs/themes/readthedocs/base.html"
+extract:
+  replace: [ ['([<]body.*$)', '\1\n<p>Kilroy was here.</p>\n'] ]
+!}