Escape ° as &#176;

These were the only remaining non-ASCII characters.
Fix invalid HTML entities &emdash; and &endash;
Escape & in R&D
Replace <P> with <DT> in the index
Add tool used to check HTML 4 validity

This was used to find some of the errors fixed previously. index.html
is still invalid, but not in a way that needs to be fixed.