New "Data" tab and rearrangement of the homepage

Following some comments from @gguerrie regarding a needed reorganisation, and after checking a bunch of open data websites, I decided to add a "Data" tab that we were missing.

• Why: If you go to an open data website you expect to find the data. That was hard to do in the current version of the website.

Overall I:

• Moved things from the documentation that were about the data.
• Added links for downloading the data and for the public notes, so that they are easier to find.
• Improved the file on how to search for the research data.

I would like to add, for each section, a "what you can do with these data" subsection in the near future.

As a bonus, I also rearranged the homepage so that people don't have to scroll to see the "get started" button.

This, of course, doesn't fix all the reorganisation needed, but I think is a good addition.