DOC: Update release document
This commit removes these obsolete sections and reference the "new-release" issue template.
Section added:
- Initial steps / Release Checklist
Section renamed:
- Announcing -> Announcing the final release
Section removed:
- Current Maintainers
- Initial steps / Increment the Version Number
- Initial steps / Merging Branches
- Creating Tarballs
- Creating Tarballs / Source
- Creating Tarballs / Documentation
- Creating Tarballs / Binaries
- Creating Tarballs / Uploading