DOC: Make section title capitalization consistent
At the moment we've got a mix of capitalization formats in the title sections of our docs. For example, in the first level of the TOC tree in http://dev.pandas.io/ we've got (not the second word first letter, `s` lower case vs `G` upper case):
- `Getting started`
- `User Guide`

And this happens all across the documentation. I think it makes more sense to just capitalize the first letter of the first word, like in `Getting started`. More a personal preference than any objective reason, but looks simpler, more standard, and we avoid things like `pandas Ecosystem` which doesn't look really great to me.

CC: @TomAugspurger @jorisvandenbossche