This commit adds an index page for the multi-file HTML version, and
fixed a couple of casing issues (significant words starting with lower
case in headings).

While researching how to get the content from index-docinfo.xml into
the output, I came across the notion of a colophon, which is a good name
for the information in that file. I have consequently changed "Legal"
(which I never liked but couldn't think of a better term for at the
time) to "Colophon".

See gh-12611