[md2orchestra] Support purpose in documentation, appinfo element

[donmendelson]

Currently in md2orchestra, documentation element is populated without its purpose attribute. The Orchestra schema enumerates these values for purpose: synopsis, elaboration, example, and display. Tablature should support those keywords as column headers and use them to populate purpose in documentation. It can continue to map "documentation" column header to a <documentation> element without purpose attribute.

There remains the issue of what to do with columns that do not match an expected attribute. If the document writer thought it was important enough to add to rules of engagement, it should be saved somewhere in Orchestra. The <appinfo> element can be used for this. Like documentation, appinfo is a child of an annotation element, and it also has purpose attribute. Any unexpected column header can be mapped to the purpose attribute of appinfo element.