Also follow through on renaming main file.

We recently renamed cug-html.html to cug.html, but some nav links
generated by htlatex, multi-page version, pointed to the original.
And avoid clash between the single- and multi-page CSS.

(see comments added to doc/Makefile)
Merge branch 'html-multi-fix' of github.com:hjoliver/cylc into html-multi-fix