Add comprehensive release process documentation

[vhaasteren]

Description:
Currently, there is no clear documentation for how to create releases for PTMCMCSampler. The release process involves both GitHub releases (which trigger automated PyPI uploads) and manual conda feedstock updates, but this information is not documented anywhere.

This makes it difficult for maintainers to perform releases consistently and creates dependency on specific individuals who know the process.

Proposed Solution:

• Create a RELEASE.md file with step-by-step release instructions
• Document the manual conda feedstock update process
• Include troubleshooting information
• Add links to the release documentation in existing docs

Benefits:

• Clear, documented process for all maintainers
• Reduced dependency on specific individuals
• Consistent release procedures
• Easier onboarding of new maintainers