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.
Problem with this change is that the next few sections have properties such as "security.authentication.handler.userBaseDn" that include the word "user". Still want this change? There is also an additional user in the start of the next paragraph...