docs: add quick API guide

[tcheeric]

Summary

• document minimal nostr-java API usage with setup and text note example
• link to new how-to guide from README

Testing

• mvn -q verify (fails: Previous attempts to find a Docker environment failed. Will not retry)

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https://chatgpt.com/codex/tasks/task_b_68a48d97715483319a3a42172a5673a9

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[Copilot]

Pull Request Overview

This PR adds documentation for basic nostr-java API usage with a minimal setup guide and example code for publishing text notes. It provides new users with a quick way to get started with the library.

• Creates a new how-to guide with dependency setup and basic event publishing example
• Adds a reference link to the new guide from the main README

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
docs/howto/use-nostr-java-api.md	New how-to guide with API setup instructions and example code
README.md	Adds link to the new API usage guide

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[Copilot]

The relay URL 'wss://nostr.example' appears to be a placeholder that won't work in practice. Consider using a real relay URL like 'wss://relay.damus.io' or add a comment explaining that users need to replace this with a working relay.

Suggested change

	Map<String, String> relays = Map.of("local", "wss://nostr.example");
	// Replace with your preferred relay if needed
	Map<String, String> relays = Map.of("local", "wss://relay.damus.io");