Add Explainer Tooltip to Energy Mix

[vkoves]

These values are really confusing, so ditching them would streamline the site without losing much information.

We should also add explainer text to the remaining tiles to explain what they mean, e.g. explaining that GHG intensity is a measure of the greenhouse gas emissions of a building divided by its square footage, making it comparable across different sizes of building.

Note: This may require adding a tooltip library so that we don't show the explainer text at all times.

[vkoves]

Here's a sample from Figma:

This library looks promising: https://atomiks.github.io/tippyjs/