Cross-promote GitLab Mattermost plugin

@twoodham @danielgruesso @ljlane As discussed yesterday, opening an issue to discuss opportunities to cross-promote the Mattermost plugin for GitLab

I'll see what mutual customer use cases there are for cross-promotion - in the meantime if you can connect me with content marketing, we can start the discussion on marketing options.

For additional context and background, please see: #28932