Create policy on how xdocs should be written

Taken from https://github.com/checkstyle/checkstyle/pull/6782#discussion_r288070286 ,

Right now, alot of xdocs are very old and we are reworking them with the 'alignment' and other issues.
There should be some policy to clarify where to put examples on how good xdocs should be written when providing updates.

Maybe this could be somewhere on the contribution page. Brief description with relevant examples for existing checks would be great to make it easier to understand relevant requirements.