The current release instructions in `CONTRIBUTING.rst` are outdated

[vhaasteren]

Proposed Solution:

• Create a RELEASE.md file with current release process
• Document the manual version management in __init__.py
• Update CONTRIBUTING.rst to reference the new documentation
• Mark outdated instructions as deprecated

Benefits:

• Accurate, up-to-date release instructions
• Clear documentation of manual version management
• Elimination of confusion from outdated instructions
• Consistent process for all maintainers