Spring Beandoc plugin index.html not created correctly

The main page for the Spring Beandoc site report appears to have two issues with it:

• the link generated in the Maven reports section of the site is named "index.html.html"
• the actual "index.html" created under site/springbeandoc is a generic Maven report template, not the main frame page created by beandoc

The fixes appear to be minor (patch attached):

• change SpringBeanDocMojo.getOutputName() to remove the ".html" extension (report generation apparently provides that)
• add an impl of MavenReport.isExternalReport() to return true.