Update Maven Central authorization and document it

[martinjoconnor]

Maven Central authorization will soon change:

Dear Maven Central publisher, 

We are making changes to the OSSRH authentication backend. For most users this should be a transparent process, and you should be able to continue to use your existing username and password to connect the Nexus UI. In case you need to update your password, please follow our documentation. 

To configure a publisher’s plugin authentication you would need to update your plugin settings to use a user token instead of the Nexus UI username and password login. 

For more information about publishing to legacy OSSRH please consult our documentation at https://central.sonatype.org/register/legacy/ 

Thank you, 
The Central Team 

Update the CEDAR settings.xml files appropriately and document.