Add a method of building a local copy of the docs

This will build the docs into a directory called docs-local, sans the blog.
Note the near-total copy/paste of the `nav` due to how mydocs does
configuration inheritance; this is fine for now.

.gitignore

@@ -110,6 +110,7 @@ venv.bak/
 
 # mkdocs documentation
 /site
+/docs-local
 
 # mypy
 .mypy_cache/