Add MM changelog to index and add 5.37 release

Today evening we will deploy Mattermost 5.37. I'd like to start the section by adding the Mattermost service changelog to the index and also document the 5.37 release.

This should then be deployed tomorrow during the course of the day.

/cc @hueser93

@jandt-desy How is the policy for changes in this project. Should I assign the reviewer role to you?