Add shared debug output functions to project for Makefiles and shell scripts

What does this MR do and why?

For: #325 (closed), and to implement pieces of !685 (closed) separately, let's:

• Add documented debug output functions that can be reused in Makefiles and shell scripts. The output functions should produce output that looks the same regardless of the source of the message.
• Update the Go-based logger so that it uses simpler colors to match the new functions.
• Implement the functions in a couple of places to thoroughly test the new functions, both locally and in CI/CD.

Validation in CI/CD pipelines:

• For make lint-markdown: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/10084480291
• For make check-global-navigation: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/10084480309
• For make check-redirect-threshold: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/10084480313

How to set up and validate locally

1. Configure a local GitLab Docs environment.
2. Run make lint-markdown to test the change to a Make target.
3. Run make check-global-navigation to test the change to a shell script.
4. Run make check-redirect-threshold to test the changes to a Make target and shell script (using fallbacks where gum isn't available).
5. Check out this branch.
6. Repeat running the Make targets.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.