docs: add draft release process (issue #583)

[leepl37]

Summary

This PR adds a draft docs/RELEASE.md describing a proposed release checklist for ldk-node, and links it from the docs (optional).

• Versioning (SemVer)
• Changelog updates
• Signed tagging
• Crates.io publish (workspace)
• Bindings / language artifacts (if applicable)
• CI/docs.rs checks
• Post-release verification
• Communication/announcement
• Future automation ideas

Why

Issue #583 requested documentation for the release process. Having a single checklist improves clarity for maintainers and contributors and reduces release friction.

Notes / Questions for Maintainers

• Are there additional steps for bindings or downstream repos we should include?
• Do we have a preferred tool for changelog automation (e.g., git-cliff/release-please)?
• Any constraints on who performs the tagging/publishing?

Testing

• No code changes. Built and ran unit tests locally to ensure no regressions in CI paths:

[ldk-reviews-bot]

I've assigned @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[tnull]

Thank you for looking into this, but documenting the release process isn't really a task that you should do as a first contribution to a repository you have no history in. Closing this.