release: bump marketing version to 1.29.4

Version 1.29.4

release: bump marketing version to 1.29.4

- Update MARKETING_VERSION to 1.29.4 in project.yml.
- Update release note index and add 1.29.4.html.
- Link version 1.29.3 and 1.29.4 release notes.
- Regenerate Xcode project with XcodeGen.

Commit: f94c3d8