feature: define Staged Configs

[mjang]

Proposed changes

Based on internal issue 1638. Checklist of what to cover (mostly in intros)

• Describe the intent of Staged Configs
  • Supports incremental edits
  • Pre-deployment
  • Allows admins to work incrementally
  • Can save a Staged Config that does not (yet) work
    • Note: invalid Staged Config(s) are OK, and can be saved
    • See intros, such as https://frontdoor-test-docs.nginx.com/previews/docs/264/nginx-one/how-to/staged-configs/add-staged-config/#overview
  • Supports "Explain with AI"
  • Can start with a config from (separate sections)
    • an Instance
    • a CSG
    • see https://frontdoor-test-docs.nginx.com/previews/docs/264/nginx-one/how-to/staged-configs/add-staged-config
  • API supports upload of config (e.g. nginx.conf)
    • Returns AI-based evaluation of the config
    • see https://frontdoor-test-docs.nginx.com/previews/docs/264/nginx-one/how-to/staged-configs/api-staged-config/, which describes use cases, with links to details in our API reference guide: https://frontdoor-test-docs.nginx.com/previews/docs/264/nginx-one/api/api-reference-guide/

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. ↩

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[JTorreG]

seeing some inconsistencies on how "instance(s)'' is capitalized

[mjang]

Will merge (after final approval) and after https://gitlab.com/f5/nginx/one/saas/control-plane/-/merge_requests/2840 is merged.

I'm assuming @vrmare 's approval based on the comments.

[mjang]

Merging based on (internal) Slack acceptance from @jasonclopper and @vrmare