Autopublishing level2/Level3 specs need to work with multiple markdown files

[kalyanaj]

Filing this based on the discussion in the W3C DT working group meeting.

Autopublishing level2/Level3 specs need to work with multiple markdown files. I don't have the complete context, but @dyladan can provide more information if needed.

From Daniel: Can run reSpec locally (the above PR runs locally fine, but preview is broken in github https://pr-preview.s3.amazonaws.com/dyladan/baggage/pull/120.html)

[plehegar]

Fixed level2 in #568 , #569, and #570

[plehegar]

level3 was never published as a Working Draft, so it cannot be published using autopublish.