feature: Add import, export, bulk management of Staged Configs

[mjang]

NOTE Approved. Hold for release on Monday May 19.

Proposed changes

Adds content / API docs for Staged Configs, specifically functionality to:

• Import
• Export
• Bulk manage several Staged Configs

Also includes corresponding API content from #489, cc @vivki

• I've added that so I can test links from https://frontdoor-test-docs.nginx.com/previews/docs/553/nginx-one/how-to/staged-configs/api-staged-config/
• Unfortunately, the link from https://frontdoor-test-docs.nginx.com/previews/docs/553/nginx-one/api/api-reference-guide/#operation/exportStagedConfig to https://docs.nginx.com/nginx-one/how-to/staged-configs/import-export-staged-config/ won't work until the changes are published. (long explanation, I'm happy to explain to anyone)

Closes #ISSUE

Checklist

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• 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
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• 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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[mjang]

Merging with the SME approval (@vivki ), who currently has "only" read access.