Add plain language description of use cases to intro guide.

[zmitchell]

From a conversation on Slack:

Touching on the use cases, Flox gives you a reproducible "environment" (think of a development environment but not just for development-time), which amounts to a certain set of packages, environment variables, setup hooks, etc. Since there's a lot of different uses for "give me the same setup in multiple places" you end up with different use cases. Want CI to look like your machine? Put Flox in CI and on your laptop. Want production to look like your machine? Put Flox on your machine and your production machine. Want all of your coworkers to have the same development environment? Put Flox on all of your machines.

Adapt this to the "Flox in 5 minutes" guide.