Resolve "Document product integration for using Big Bang pipeline"

Changes

Changing developer docs from using .gitlab-ci.yaml in packages to calling the pipeline in the repo's settings.

For Issue

Closes #840 (closed)

A new issue for creating a full Big Bang pipeline for packages to run should be created as per https://repo1.dso.mil/platform-one/big-bang/bigbang/-/issues/840#note_478046