DeveloperGuide: use proper syntax for figure captions
DeveloperGuide: use proper syntax for figure captions

The DeveloperGuide manually numbers each figure.

Such manually numbering of figures is error prone and leads to a problem
where inserting a figure between two other figures causes a "snowball"
effect where all subsequent figure numbers needs to be updated, which
pollutes the commit diff.

As such, let's switch to using asciidoctor's automatic figure numbering
instead, which kicks in when we switch to using the proper syntax for
figure captions -- by defining the caption text as the title of the
block image[1].

As a side benefit of using the figure caption syntax, we can now also
use the cross reference syntax to reference other figures, which will
give us the desired behavior of referencing that figure with the correct
figure number.

One downside of this approach is that the automatic figure numbers are
not hierarchical, i.e. they just go 1, 2, 3... without taking the
section numbers into account. However, the upside of not having to
manually update figure numbers is much more valuable.

[1] http://asciidoctor.org/docs/user-manual/#images

damithc/addressbook-level4,damithc/addressbook-level4,damithc/addressbook-level4,se-edu/addressbook-level3