git-debrebase(1): Make this manpage a less intimidating entry point

git-debrebase(1): Make this manpage a less intimidating entry point

- Nicer short description right in the first section.

- Don't use the term 'delta queue' until it's needed.

- Don't talk about other manpages so early.

- Cater for people who type `man git-debrebase` looking for a
  barebones tutorial, not (just) a command line reference.

  - That tutorial should minimise references to dgit, especially since
    users might want to use git-debpush to upload.

Thanks to Sam Hartman for the user feedback which prompted this and
Ian Jackson for feedback on the content of "QUICK REFERENCE".

Closes: #926656
Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>