Markdown-many processes several documents simultaneously as if it
was processing one large document. Its chief purpose is to make it easy to
create inter-document links. Markdown-many takes as input

* pairs - a list of lists where each sublist contains the markdown
file to be processed as input in its first element and the name of
the file to be produced as the output.
* :format - a keyword argument specifying the kind of output document
to produce
* :additional-extensions - a list of extensions that should be active
both while parsing and rendering.
* :render-extensions - a list of extensions that should be active
during rendering.

Here is an example: suppose document-1.md contains

# Document-1

See document-2 for details.

and document-2.md contains

# Document 2

Document-1 provides an overview.

Getting these links to work using only Markdown will require added explicit
reference link information that will be tied to the file names. Markdown-many,
on the other hand, will automatically combine the link information and
processes it automatically.