Add mdbook setup for documentation

• Serve an mdbook setup on https://alpm/archlinux.page
• Move all rustdoc to its own namespace
• Move specification info from top-level README to mdbook setup for nicer link setup

Fixes #76 (closed)

resources/docs/src/specifications/concepts.md

+## Concept specifications
+
+Specifications for common fields, composite types and concepts are provided by the [alpm-types] [component] in its `resources/specification/` directory.
+
+The specifications are provided as specially formatted markdown files which can be rendered to [section 7] manpages.
+
+[alpm-types]: ../alpm-types
+[component]: ../#components
+[section 7]: https://en.wikipedia.org/wiki/Man_page#Manual_sections