chore: use "pr:" prefix for changelog labels; elaborate on labeling process

[Josh-Cena]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

The name "tag:" is a bit confusing, because "tag" also refers to the git tags for each release. Using "pr:" eliminates the ambiguity.

I've also elaborated in the release docs about how labeling should happen.

Test Plan

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

[netlify]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 67	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 85	🟢 99	🟢 100	🟢 100	🟢 90	Report

[slorber]

👍