Move images into continuum-docs module

Pages in the continuum-docs module are using images stored in the continuum-site module.

We should keep docs-related images in the continuum-docs module with the versioned documentation.

For example:
http://maven.apache.org/continuum/docs/1.1/user_guides/notification/index.html
links to src="./../../../../images/notifiers-link-project-group.png" which is
http://maven.apache.org/continuum/images/notifiers-link-project-group.png
in .../continuum/trunk/continuum-site/src/site/resources/images