[WIP] [Doc] OpenAPI spec

[kaisecheng]

Release notes

What does this PR do?

Why is it important/What is the impact to the user?

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• CLoses: [Logstash API] Complete API doc migration to OpenAPI spec #16967

Use cases

Screenshots

Logs

[mergify]

This pull request does not have a backport label. Could you fix it @kaisecheng? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.
• If no backport is necessary, please add the backport-skip label

[github-actions]

📃 DOCS PREVIEW ✨ https://logstash_bk_17292.docs-preview.app.elstc.co/diff