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.

Makefile

@@ -15,6 +15,8 @@ docs-serve:
 	mkdocs serve
 docs-build:
 	mkdocs build
+docs-local-build:
+	mkdocs build --config-file mkdocs-local.yml
 docs-deploy:
 	rm -rf .screenshot_cache
 	mkdocs gh-deploy