Adding small grid to home page and gallery page

This PR implements in the github.io/Jekyll framework a similar grid for the landing page as it currently stands in readthedocs.org and adds a page named "Gallery" (grid.html) to display different images that point to additional notebooks or resources (not included in the page, typically external). Because, like the rest of the site, it is Jekyll compatible, it can be edited from a machine or through the browser, and Github will build the site before it goes live.

@sjsrey is this something similar to what you had in mind?