Add API docs contribution guidance under extend/contribute

extend/contribute/api-docs/checklist.md

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+---
+navigation_title: Checklist
+---
+
+# API docs checklist
+
+Use this checklist to verify the quality, completeness, and consistency of your API docs contributions.
+
+## Structure, organization, and metadata
+- Include required [OpenAPI document info](./organize-annotate.md#add-open-api-document-info)
+- Include [OpenAPI specification version](./organize-annotate.md#add-openapi-specification-version)
+- Define unique [operation identifiers](./organize-annotate.md#add-operation-identifiers) using camelCase
+- Use consistent [tags](./organize-annotate.md#group-apis-with-tags) to group related operations
+- Document [API lifecycle status](./organize-annotate.md#specify-api-lifecycle-status) (availability, stability, version information)
+- Mark deprecated APIs and properties with appropriate notices
+- Document [required permissions](./organize-annotate.md#document-required-permissions) for each operation
+
+## Content quality and completeness
+
+- Write clear [summaries](./guidelines.md#write-summaries) (30 characters max, start with a verb)
+- Write detailed [descriptions](./guidelines.md#write-descriptions) for operations, parameters, properties, tags etc.
+- Document all [path parameters](./guidelines.md#document-path-parameters) with constraints and formats
+- Provide descriptions for non-obvious [enum values](./guidelines.md#document-enum-values)
+- Specify [default values](./guidelines.md#set-default-values) for optional parameters
+- Include realistic [examples](./guidelines.md#add-examples) with helpful descriptions
+- Add [links](./guidelines.md#add-links) to related operations and documentation
+
+## Quality assurance
+
+- Preview your changes locally before submitting (see [Contribute locally: Elasticsearch quickstart](./quickstart.md))
+- [Lint your API docs](guidelines.md#lint-your-api-docs) to identify and fix issues
+- Check all links to ensure they work correctly
+- Ensure examples are realistic and error-free