Include 5.10.4 release notes

[marcphilipp]

Overview

The 5.11.1 release notes should contain the 5.10.4 release notes. Therefore, we need to copy them from the releases/5.10.x branch to the releases/5.11.x branch after releasing 5.10.4

Deliverables

• Copy 5.10.4 release notes
• Include them in the release notes index file