SPARK-2830 [MLlib]: re-organize mllib documentation

As per discussions with Xiangrui, I've reorganized and edited the mllib documentation.
It does seem odd to talk about linear regression, lasso and ridge regression as distinct algorithms, but talk about linear SVMs and logistic regression as single algorithms and not mention different regularizers.  That being said, this is often done in practice.  We could add the following sentence to clarify this point, e.g.,  "...: linear support vector machine (SVMs) and logistic regression.  We support both L1 and L2 regularization for these methods."