Set up a `ci-generate.yaml` workflow

[benfrankel]

When a user generates a new repo starting from this template, their initial commit should pass CI. We should make the ci-generate.yaml workflow test precisely this:

• Check out the template
• Run cargo-generate to generate a new repo
• Check that generated .template files match the file they'll replace (with a list of files to exclude from this check, such as README.md.template)
• Run CI commands in the generated repo

cargo-generate.toml should exclude ci-generate.yaml from generated repos.