Renaming `sub-value` to `operating principle`

Why is this change being made?

During the content discussion on MECC, it was noted that GitLab sub-value's are actually operating principles. Renaming to reflect this observation.

The goal is for sub-values to be actionable, and to guide behavior. Being clear about their function as an operating principle allows them to be more easily lived by GitLab team members, and less esoteric to other organizations who seek to learn from GitLab's Remote Playbook and MECC management philosophy.

I'm intentionally excluding .js files as well as READMEs and blog posts from the rename as I was unsure if they should be included.

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