DOC: replace long table of contents on home page with main links
This is a proposal to remove the long table of contents on the documentation home page and replace it with a highlight of the main parts of the documentation.

Reasons: I don't think the long table of contents resulted in a very attractive landing page of the docs. And further, it was also a "manual" table of contents which would therefore easily get out of date. And with the new content, we can also at the same time give some explanation on the organization of the docs.

So would like to get feedback on the general idea. The wording in the different boxes can certainly still be improved. And we could also make similar boxes to point to eg the installation guide or for the different "getting help" links. 

It's using some bootstrap elements ("cards") that can be used now since the sphinx theme is based on bootstrap (and all credit to @stijnvanhoey for the html/css part, I re-used some content of what he worked on for the getting started guides, a PR for that is coming shortly). Screenshot of how it currently looks below, and a live version can be checked here: http://jorisvandenbossche.github.io/example-pandas-docs/html-doc-home/

![image](https://user-images.githubusercontent.com/1020496/72721384-35a1c880-3b7c-11ea-87af-eb26bff4f3db.png)
