Fix up URL references to the Handbook in the 4.6-RELEASE and 5.0-DP1
release documentation.

With one exception (the 4.6-RELEASE announce.sgml), these are all
generated files. Usually I would be opposed to editing the generated
output, except that we're already getting bug reports resulting from
changing the way we refer to the Handbook.

Only doing this for 4.6-RELEASE and 5.0-DP1 now because these are the
latest active "releases".

Suggested by: nik