[INT-160] Add CI job to publish indigo website to indigo-lang.gitlab.io

We want to publish indigo docs to indigo-lang.gitlab.io, to use this domain name we have to do publishing from a separate project: https://gitlab.com/indigo-lang/indigo-lang.gitlab.io. This MR adds a CI job to trigger pipeline in that project and pass built website to it. The pipeline in that project downloads built website as an artifact and publishes it to gitlab pages

Downstream pipeline MR: indigo-lang/indigo-lang.gitlab.io!1 (merged)

We haven't set up CI runner for the downstream project yet (I have been using a temporary runner for testing), so this shouldn't be merged until we have

Example of the job running can be seen here: https://gitlab.com/morley-framework/morley/-/pipelines?ref=zhenya%2Fint160-publish-indigo

Published website: https://indigo-lang.gitlab.io/