docs: add the package-template-repo in more places

[leeederek]

Where did you see the issue in our documentation?

https://docs.kurtosis.com/

Issue description

A user did not know the package-template-repo existed for them to use when writing new packages. This Github issue tracks the effort to make the package-template-repo more visible in our readme and docs

https://github.com/kurtosis-tech/package-template-repo