Doc: Further improvements for IntervalIndex and Interval
- [x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This follows #17706. Further changes are:

* Taken text from the whatsnew for v0.20 and use it in advanced.rst to give more explations to ``IntervalIndex`` and ``Interval``.
* in interval.pyx, the doc string "Attributes" didn't parse properly, because sphinx doesn't like having "left" and "right" on the same line. They're now on separate lines and the section renamed to 'Parameters'. See current here and compare to source: http://pandas-docs.github.io/pandas-docs-travis/generated/pandas.Interval.html#pandas.Interval.
* some examples are added to ``Interval``
* the warning text on ``IntervalIndex`` is now parsed properly.
* The "See Also" had become quite large, felt it becomes more reader-friendly to combine the ``from_*`` metods into one line.
* added ``cut`` and ``qcut`` to 'See also'

This will conclude my contributions to ``IntervalIndex``.