Improve plugin documentation on http://maven.apache.org/plugins/index.html.

On http://maven.apache.org/plugins/index.html, the following text is
available:

"To see the most up-to-date list of available plugins, browse the
Maven 2 plugin repository at http://www.ibiblio.org/maven2/."

However in this folder you have:
1. maven-plugins : apparently contains only M1 plugins
2. plugins : contains base M2 plugins and one codehaus plugin
3. plugin : empty, so can be deleted
4. maven-torque-plugin : only metadata files, so can be deleted
5. maven-xdoclet2-plugin : empty, so can be deleted

To avoid confusion for novice M2 users, I think it's best to:

a. delete the unnecessary folders (3, 4, 5)

b. document in the website that

"http://www.ibiblio.org/maven2/plugins"

should be verified for the latest versions of M2 plugins instead of

"http://www.ibiblio.org/maven2".

c. add a remark to state that

"http://www.ibiblio.org/maven2/maven-plugins"

is only for M1 and should be ignored for M2.