Move graphql API section to separate README

[aditya1702]

What

• Move the API reference README section to another README in /internal/serve/graphql
• Update the API documentation to match the current schema.

Why

• The landing page README is very long and this provides a better developer experience to directly reference the API.

Known limitations

N/A

[Copilot]

Pull request overview

Moves the API reference documentation out of the root README into a dedicated GraphQL/API reference README, and refreshes the API docs to better match the current GraphQL schema.

Changes:

• Added a new internal/serve/graphql/README.md containing the full API reference (REST transaction endpoints + GraphQL schema usage).
• Removed the inlined API reference sections from the root README.md.
• Updated the root README Table of Contents to link to the new API reference document.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
internal/serve/graphql/README.md	New, dedicated API reference document for transaction endpoints and the GraphQL API schema.
README.md	Removes inlined API reference content and links readers to the new API reference README.

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