docs: Document the event schema for all contract events with field descriptions

[FaveTeamz]

Summary

The contract emits structured events for all state transitions (apply, withdraw, assign, complete, revoke, initialize). The event schema is not documented anywhere outside the source code, making it hard for integrators to build event-driven applications on top of the contract.

Problem

An integrator building a notification service on top of Soroban event streams must reverse-engineer the schema from src/lib.rs. There is no canonical reference for field names, types, and which events carry which fields.

Acceptance Criteria

• Create docs/event-schema.md with a table per event type:
  • Event name, emitted by function, fields (name + type + description), example XDR value
• Cover all 7 event types: Initialize, Apply, Withdraw, Assign, Complete, Revoke, Upgrade
• Include a code sample showing how to subscribe to events via Horizon SDK
• Add a JSON Schema file at docs/schemas/contract-events.json for automated validation
• Link from docs/api-reference.md and the README

[Daveeee-tech]

@Daveeee-tech has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi maintainer I would love to solve this issue

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[Achievers-sketch]

@Achievers-sketch has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Document all seven contract event schemas with field types, descriptions, XDR examples, subscription guidance, and JSON Schema validation, then link the documentation from the API reference and README.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Achievers-sketch to this issue.

[soter-fashion]

@soter-fashion has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

hello maintainer i will like to work on this issue

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