Prettify the mod index with tiles and graphics

[simonbowly]

Possible examples to follow:

• YALMIP examples - unfortunately appears to be jekyll/markdown/ruby based, but it looks cool
• sklearn - though their tiles are categories

[simonbowly]

sphinx-design provides a nice reflow grid layout

[simonbowly]

489ab38 sets up a gallery page using sphinx-design. Now just need nicer icons ... @venaturum any inspiration?

[venaturum]

As in an icon per mod?

[simonbowly]

Maybe the current set are ok? We're just re-purposing figures from the results in the docs, I was thinking more along logo lines, but the current iteration does look quite nice already:

[rluce]

I think we're good here, let's rather create follow-up issues should somebody (yes, I'm talking about you, @venaturum) come up with better stuff; closing.