doc: Updates to the first section of the Dev Guide

doc: Updates to the first section of the Dev Guide

* Move the Nexus-lite example at the end of the section. Typically
  you exlain things first, then show an example, not the other way
  around.
* Apparantly the Nexus code itself was never merged in the tree.
  Remove the references to this code (hopefully the contents of
  the doc is still useful by itself).
* Wrap non-list text to 80 columns. Makes the text files and git
  diffs easier to read.

Change-Id: I790f8087a792ed66fae8f2d9511bf76d448ffbed
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/41470
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>