ra/next: Allow alternate resource agent directory locations

This improves the text of the former "Paths" section, as two new sections
"The Resource Agent Directory" and "The Resource Agent Directory Tree".

It mentions the historical discrepancy between the standard location of the
resource agent directory and the widely implemented location, and allows for a
divergence between strict compatibility with the 1.0 standard (using only the
standard location) and wider compatibility (using a configurable location).