Replace multiple images with a single image

The multi-image layout renders correctly on github.com but does not
render correctly on plugins.jenkins.io.  The technique that was being
used is uncommon for Jenkins plugins.  Much better to use the layout of
a single image that is approximately the same width as the other images
included in the page.

README.adoc

@@ -6,13 +6,7 @@
 [#introduction]
 == Introduction
 
-[.float-group]
---
-[.text-center]
-image:https://jenkins.io/images/logos/jenkins/jenkins.png[Jenkins logo,height=192,role=center,float=right]
-image:images/signe-1923369_640.png[plus,height=64,float=right]
-image:https://git-scm.com/images/logos/downloads/Git-Logo-2Color.png[Git logo,height=128,float=right]
---
+image::images/jenkins-and-git.png[Jenkins and Git]
 
 The git plugin provides fundamental git operations for Jenkins projects.
 It can poll, fetch, checkout, branch, list, merge, tag, and push repositories.