feat: Render top-level docs using mdbook
Rely on mdbook to render top-level documentation in a concise manner.
Using the build-book just recipe the mdbook (including adhoc addition
of js requirements for nermaid graphs) is built from a preselected list
of documentation files.
Afterwards all Rust documentation is collected in a rustdoc
subdirectory of the shared documentation output directory.
The watch-book and serve-book just recipes enable a worklow in which
relevant files are watched and the output dir is continuously served
using an adhoc webserver.
Fixes: #124 (closed) Signed-off-by: David Runge dvzrv@archlinux.org