Refactor security documentation for 2.5.2

Some refactoring for the Security documentation. Gives better distinction between the configuration and the usage.

Known issues:
1. The three tables under ``LDAP Authentication`` are formatted differently. I can't figure out how to get them to align with each other.
2. Some of the links are broken.
3. As discussed on Hipchat, we may want to consider pulling ``Security`` out one more level.
To maintain consistency, this is the refactoring suggested:

Add back the ``Custom Authentication`` section to the bottom of the page. The content can be:

"To use a Custom Authentication mechanism, set ``security.authentication.handlerClassName `` in ``cdap-site.xml`` with the custom handler's classname. In addition, any properties set in ``cdap-site.xml`` are available through a ``CConfiguration`` object and can be used to configure the handler. To read about how to have a Custom Authentication Mechanism go to [Link to the other page's same section]."