Reworked chapter about file-based configurations in user guide.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1593827 13f79535-47bb-0310-9956-ffa450edef68
Merged the document about file systems with the chapter for file-based configurations.

It makes sense to have this information in a single chapter of the user guide.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1593828 13f79535-47bb-0310-9956-ffa450edef68