README and general information in oak-doc refers to svn

mreutegg, following the move to git, the README.md of oak-doc also needs a bit of attention. it still refers to svn checkout/commit.

while searching for outdated security related links, additionally spotted:

• participating.md : The latest Oak sources are available for checkout from [svn][1]
• roadmap.md : releases directly from [trunk](https://svn.apache.org/repos/asf/jackrabbit/oak/trunk/):
• branching.md : {{-DbranchBase=https://svn.apache.org/repos/asf/jackrabbit/oak/branches }}
• pom.xml : <url>scm:svn:https://svn.apache.org/repos/asf/jackrabbit/site/live/oak/docs</url>