Reorganized docs a bit.  Added migration guides.

**Q**: Do we want to say more for the 1.3 -> 1.4 migration guide for ```spark.ml```?  It would be a lot.

CC: mengxr

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #6897 from jkbradley/ml-guide-1.4 and squashes the following commits:

4bf26d6 [Joseph K. Bradley] tiny fix
8085067 [Joseph K. Bradley] fixed spacing/layout issues in ml guide from previous commit in this PR
6cd5c78 [Joseph K. Bradley] Updated MLlib programming guide for release 1.4