Remove obsolete user's guide html pages.

ParaView's embedded help system will no longer have the user's manual in
it. This removes all those html pages and updates the index page to
point users to the pdf version of the user's manual.

Change-Id: I8bbf3c37f51e36f067fea161c74128cb7ea1ba39
Merge topic '14720-RemoveQtWebkit' into next

bfe41e00 Remove obsolete user's guide html pages.