Docs: define use of footer for output DYN files.

Docs: define use of footer for output DYN files.

doc/Programming-Guide/doxygen.footer.dyn is an empty file because we do
not need anythign as a footer. But we do need to use it because doing so
will suppress teh default "</body></html>" string being appended to the
output file, which breaks out static site generation since .dyn files
have our own squid-cache.org footers automatically appended.