First SVG graphic

Please find in the attachment a SVG file which shows the internal
structure of PG's pages and a patch to insert it to "storage.sgml".
Because this is the first graphic for our documentation many additional
work must be done. SvgHandling.pdf explains how to create graphics and
integrate them into the textual description, which definitions are
actually missing (style guide?), and what problems may occur.

We shall create two new directories: doc/src/sgml/svg (as source
directory for all svg files) and doc/src/sgml/html/svg. The patch for
Makefile is also attached - HTML, PDF, and EPUB are tested. There is a
conceptual problem with single-page HTML. The SVG files keep separate
and I don't know how to include them into the generated single file
"postgres.html".

@Committers: If you tend to accept this patch, I will start to create
wiki-pages and additional documentation pages to describe the
proceeding. If you reject the patch, please give me a justification.

Kind regards

Jürgen Purtz