DOC: start using new bootstrap-based sphinx theme
closes https://github.com/pandas-dev/pandas/issues/15556/

I want to propose that we start using the new bootstrap-based theme that is being developed at https://github.com/pandas-dev/pandas-sphinx-theme/ for the dev docs. 
This way it will already be used for the docs at https://dev.pandas.io/docs/ so it can get some exposure before the next release.

How I did it in this PR is to install it from git master https://github.com/pandas-dev/pandas-sphinx-theme/ instead of moving the actual source into the pandas repo. I would prefer doing it like this for now, as that makes it easier to further iterate on the theme (the other repo is set up with a faster doc build (disabled ipython directive + smaller api) and automatic preview using doctr on travis). In a later stage, we can still move it here if we want (or move the pandas-specific customizations here).

A preview (of a subset, not all API pages) can be seen at https://dev.pandas.io/pandas-sphinx-theme/  

There are still several "must todo's" to get the theme in a decent enough state for a release. There are some open issues on the theme repo, but I can also open an issue here to keep track of those.  
And of course, feedback on the theme is very welcome.

I made two other changes:

- moved the install.rst into the getting_started directory (so it is not a top-level navigation item). Given that the new website will have a page with a quick install linking to this more advanced install page, I think that is fine (and also, on a reworked home page of the docs it could also get a prominent place without being in the navigation bar)
- Idem for the ecosystem page, as this will also get more exposure on the new markdown website (we actually need to remove that page / reconcile it with the website, but let's leave that for another PR)