Add comprehensive release process documentation and update outdated instructions

[vhaasteren]

Summary

This PR adds detailed documentation for the enterprise_extensions release process and updates outdated release instructions in CONTRIBUTING.rst.

Changes Made

• Added RELEASE.md: Comprehensive release documentation including:
  • Semi-automated release process (manual version update + automated PyPI upload)
  • Manual version management in __init__.py
  • Trusted publishing setup
  • Troubleshooting and release checklist
• Updated CONTRIBUTING.rst:
  • Added "For Maintainers" section with link to release documentation
  • Marked outdated bumpversion/Travis instructions as deprecated
  • Added note directing users to new documentation

Release Process Documented

• Manual version updates in enterprise_extensions/__init__.py
• Automated PyPI uploads via GitHub Actions with trusted publishing
• Manual release notes updates in HISTORY.rst
• GitHub release creation process

Benefits

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• Elimination of confusion from outdated instructions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