Verifying and sharing custom override yaml from within development-maintenance.md is difficult

Problem

Often when I have trouble getting GitLab to work on my local k3d_dev cluster I seek help from fellow BigBang engineers. We usually end up trading yaml override incantations. It is difficult to reproduce each others' setups as long as we're leaning heavily on local-only YAML override files.

Proposal

1. Use gitlab-hosted overrides for our local work, i.e. -f https://repo1.dso.mil/big-bang/bigbang/-/raw/master/tests/test-values.yaml instead of -f ../bigbang/tests/test-values.yaml. This helps ensure we're always using the latest-merged YAML values and it also helps teammates see exactly what values we're using.
2. Where we need to use custom yaml configs for local testing of renovates and such, we should check those configs in to the relevant repositories as .yaml files. Specifically, the 100+ line custom YAML config in this repo's docs/DEVELOPMENT_MAINTENANCE.md could be moved to docs/development-overrides.yaml.