User Guide Tuning

Adjust the heading levels in the writeups for the input, loader, security and unit testing libraries, to match the other library writeups.
Specifically, the class reference section in each of these was appearing as a sub-level in the toctree, when it should be at the same level.
Signed-off-by:James L Parry <jim_parry@bcit.ca>
Merge pull request #3429 from jim-parry/userguide/fix-reference-level

User Guide Tuning