Update CHANGELOG.md (layout and links to MRs)

5 jobs for releases/2.6
Status Job ID Name Coverage
  Test
canceled #23560
duneci
debian:10 gcc:c++17

canceled #23562
duneci
debian:9--clang

canceled #23561
duneci
debian:9--gcc

canceled #23564
duneci
ubuntu:16.04--clang

canceled #23563
duneci
ubuntu:16.04--gcc