Add project url to our indices

[carolynvs]

The mixin and plugin indices should have a project URL so that people can quickly find the docs it. For example, the helm3 mixin can link to https://porter.sh/mixins/helm3, and otherwise people can link to their GH repo.

The index.json files in this repo should be updated to include a new field "projectURL" with links to the github repository.

See our Contributing Tutorial and New Contributor Guide for help getting started contributing to Porter.