Update the ruby post-processor for AsciiDoc guides so
that any existing dbhtml processing instructions are
identified, which causes a dbhtml PI to be injected
immediately after the <book> tag.
This allows AsciiDoc-based guides to specify which
content should appear as index.html. The default
index.html is generated when the dbhtml PI is not
detected, and consists of the copyright statements.
This includes the necessary update to the DVCS
guide to make the Introduction chapter appear in
index.html.