Doc: Added design docs for multiple modules

Design doc added for

* Multinomial logistic regression

* Linear Systems

* Standrd errors for marginal effects of multinomial logistic regression
Upgrade: Bug fix for installation on Mac

Bug: When there is no /usr/local/madlib, installation fails due to the
non-existence of the folder when running preflight script

Fix: Detect whether the folder exists first in both preflight and
postflight scripts
Documentation: Relocate technical background for linear and logistic

- Reference technical doc from about section
- Replace non-working example with one from tests for logistic
- Add doxygen @background alias
- Relocate technical background