Add preliminary documentation for Beaverfiles
Switch block manifests to YAML

Modify the documentation so that block manifests use YAML instead of
INI format.  This is more suitable for us because it allows us to a lot
of nesting with lists and dictionaries, necessary for specifying
multiple modes with a various parameters.  We could do this with INI
sections, but it’s messy, and we can’t nest them.  We’ll be adding more
stuff to these files later, and YAML just keeps it cleaner.