Create maintainership process for gitlab-elasticsearch-indexer

I'm working on a maintainership process for the gitlab-elasticsearch-indexer project

• implement danger simple_roulette in the project gitlab-org/gitlab-elasticsearch-indexer!153 (merged)
• create a trainee maintainership template for others to use to become maintainers gitlab-com/www-gitlab-com!109187 (merged)
• backend team members (existing and new) should identify as reviewers in team yml
• add becoming trainee maintainer to the team onboarding issue for backend !16 (merged)
• add maintainership process to project page gitlab-org/gitlab-elasticsearch-indexer!154 (merged)
• invite other team members who know golang to become reviewers or trainee maintainers