Use mkdocs for building the documentation site. (#456)

mkdocs.yml

@@ -0,0 +1,18 @@
+site_name: docker-mailman
+site_description: Container images for GNU Mailman
+
+theme:
+  name: material
+
+repo_name: maxking/docker-mailman
+repo_url: https://github.com/maxking/docker-mailman
+use_directory_urls: true
+nav:
+  - Home: "index.md"
+  - Mailman Core: "core.md"
+  - Mailman Web: "web.md"
+
+markdown_extensions:
+  - toc:
+      permalink: true
+      toc_depth: 3