Remove RELEASE.md documenting NuGet release process

The `RELEASE.md` file has been removed from the repository. This file previously outlined the release process for the `XperienceCommunity.Sustainability` package, including steps for configuring Trusted Publishing, updating version numbers, creating GitHub releases, and troubleshooting.

Its removal suggests that the release process documentation is no longer required in the repository, potentially due to a change in the release strategy or migration of this information to another location.

Author: liamgold