docbook-xml: Switch from debian packages to upstream docbook sources

This patch changes the docbook-xml distribution package source from
debian to docbook which aligns the recipe with the source
originator. Other changes made to the recipe include:

This patch resolves [YOCTO #13270].

- removal of version 4.0 given docbook no longer distributes version
  4.0
- simplification of the header description.
- inline comments to communicate the rationale behind gathering
  multiple versions and LICENSE file handling.
- minor ordering changes to align with styling conventions and better
  context to better communicate use
- Added comment about magic XMLCATALOGS variable. This variable and
  subsequent inherit might be better placed in recipes that depend on
  docbook-xml but leave it for now to not cause a patch landslide.
- Modify the related docbook-xml patch to remove docbook-xml-4.0 since
  it is no longer provided.
- Put BBCLASSEXTEND to end of recipe per RP note on convention

Signed-off-by: Richard Elberger <rich@richelberger.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>