Convert PAIP.txt to PAIP.md

[ianchanning]

If you convert the PAIP.txt to PAIP.md (I think it effectively is markdown), then it can be used as page references to the book, because it has page markers <a id="page-..."></a>.

This becomes useful for such things as the PAIP Retrospective Lessons that include page numbers.

I created an example https://github.com/ianchanning/paip-lessons that uses the lessons with links to a PAIP.md