$ mix dextrin.docs SCHEMAS [--out DIR] [--title TITLE]
Reads every .dxns file under SCHEMAS, its subdirectories included,
and writes their documentation to DIR (doc/schemas when not given):
an index.html, a page for each namespace, and style.css. TITLE
names the site; it is SCHEMAS's directory name when not given. See
Dextrin.Schema.Docs.