feat: Add closed contribution process

[ADubhlaoich]

Proposed changes

This commit adds a new document to explain the closed contribution process. As part of the changes, related content is updated from the existing documentation specific to F5 employees, decoupling this particular use case from general guidance.

At a future point, both documents may be reorganised into alternate files as part of peripheral efforts to document ways of working.

Checklist

Before merging a pull request, run through this checklist and mark each as complete.

• I have read the contributing guidelines
• I have signed the F5 Contributor License Agreement (CLA)
• I have rebased my branch onto main
• I have ensured my PR is targeting the main branch and pulling from my branch from my own fork
• I have ensured that the commit messages adhere to Conventional Commits
• I have ensured that documentation content adheres to the style guide
• If the change involves potentially sensitive changes¹, I have assessed the possible impact
• If applicable, I have added tests that prove my fix is effective or that my feature works
• I have ensured that existing tests pass after adding my changes
• If applicable, I have updated README.md and CHANGELOG.md

Footnotes

1. Potentially sensitive changes include anything involving code, personally identify information (PII), live URLs or significant amounts of new or revised documentation. Please refer to our style guide for guidance about placeholder content. ↩

[github-actions]

✅ Deploy Preview will be available once build job completes!

Name	Link
😎 Deploy Preview	https://frontdoor-test-docs.nginx.com/previews/docs/316/

---

[mjang]

We needed this, thanks @ADubhlaoich . I appreciate your attention to these comments.

[mjang]

Changing review from "Request changes" to "Comment"