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.
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).
