Add RELEASE.md documenting the release process

[ennosigaeus]

Summary

• Documents the previously-undocumented STIX-GSW release process: version bump + metadata sync (stix/VERSION.txt, .zenodo.json, CITATION.cff), tagging, gh release create conventions, and Zenodo DOI verification.
• Adds a section on distributing to SolarSoftware (SSW), generalized without server hostnames/paths since this is a public repo.
• Written while cutting the v0.6.2 release, to make the (fully manual, no-CI) process repeatable for future maintainers.

Test plan

• N/A (documentation only)