The guide describes how to boot an instance from a volume, as well as
how to attach and set up additional volumes using terraform and
cloud-init.

This CL further adds the mkdocs-macro-plugin and a custom macro
`code_include` to load a code snippet from an external file. All the
code snippets from the new guide are extracted into separate files,
which means they can be linted and formatted using the usual tools. The
custom macro was needed because the pymdown snippets didn't work. The
custom macro also derives the code language from the file extension. New
file types must be added manually.