Update Doc generation to use GitHub Flavored Markdown

[theory]

Update bin/gendoc to produce GitHub Flavored Markdown instead of MultiMarkdown. Or, rather, change the doc comments in the sql/*.sql files to use GitHub Flavored Markdown:, especially:

• Switching from four-spaced indents to code fences for examples
• Replace definition lists with something else

Should then be able to change the contents of DB-API to use the markdown output rather than convert it to HTML.