restructure "Flavors of Pure Data" for clarity

1. move "Flavors of Pure Data" below "Goals" section
2. change heading to something like "Purr Data and the Pd Language" or "Purr Data's relationship to Pure Data."

Since all the headings are for a README that lives in the Purr Data repo, a heading that starts with "Pure Data" is awkward and can seem like a typo.