Add landing page images

[dwhswenson]

• [x] Image for "Open Source"
• [x] Image for "Binding free energy"
• [x] Image for "Drug design and discovery"

First thoughts are:

• Open source: that cheesy low-f image of a screen of code (maybe try to make it Python code, not HTML like everyone else does 😂)
• BFE: Maybe a thermodynamic cycle similar fig 2 in the livecoms best practices
• Drug design and discovery: Maybe one of our atom mapping visualizations for something interesting-looking?

[dwhswenson]

Solved in #49 and #53.