Cleaned up basic-infection. Fixes #239.

[RobertJacobsonCDC]

Moved disease_model from the docs to examples and made a symlink in the docs.

The disease_model example replaces basic-infection; basic-infection is still used as a benchmark.

It is probably wise to wait to merge this until #214 and #215 are merged, because this PR will have to modify changes to workflows modified by those PRs.

[RobertJacobsonCDC]

Some frustrating issues:

• Because of limitations in Cargo, we cannot run disease_model with cargo run --example disease_model, because disease_model is itself structured as a package.
• Clippy breaks if I include disease_model as a binary in Cargo.toml ([[bin]] name = disease_model). In particular, it doesn't "see" dev dependencies.
• Clippy goes bananas on the benchmark's test modules. I added allow to everything in the test modules to get them to pass because of false positives.

I think structuring the project will fix these issues, but I'd have to try it to be sure.