[GRAPHQL] Add GraphQL API documentation with creation and testing guides

[Shamly-Shanawaz]

Purpose

This PR adds comprehensive documentation for GraphQL APIs in Bijira, including creation methods and testing guides. The documentation follows the same structure and patterns as existing WebSocket and HTTP API documentation.

Issues

Fixes https://github.com/wso2-enterprise/apim-saas/issues/1475

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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All committers have signed the CLA.