add example for running flux

[charlesfrye]

thanks @Arro for the initial version!

Type of Change

• New example
• Example updates (Bug fixes, new features, etc.)
• Other (changes to the codebase, but not to examples)

Checklist

• Example is testable in synthetic monitoring system, or lambda-test: false is added to example frontmatter (---)
  • Example is tested by executing with modal run or an alternative cmd is provided in the example frontmatter (e.g. cmd: ["modal", "deploy"])
  • Example is tested by running with no arguments or the args are provided in the example frontmatter (e.g. args: ["--prompt", "Formula for room temperature superconductor:"]
• Example is documented with comments throughout, in a Literate Programming style.
• Example does not require third-party dependencies to be installed locally
• Example pins its dependencies
  • Example pins container images to a stable tag, not a dynamic tag like latest
  • Example specifies a python_version for the base image, if it is used
  • Example pins all dependencies to at least minor version, ~=x.y.z or ==x.y
  • Example dependencies with version < 1 are pinned to patch version, ==0.y.z