Add commit tags to handbook

We have some new tags that will pull commit specific blog posts in their post event landing pages. This information needs to be added to the handbook here: https://about.gitlab.com/handbook/marketing/blog/#tags

tag: brooklyn-2019 Pulls blog post here https://about.gitlab.com/events/commit/brooklyn/

tag: london-2019 Pulls blog post here https://about.gitlab.com/events/commit/london/

tag: sanfrancisco-2019 Pulls blog post here https://about.gitlab.com/events/commit/sanfrancisco/ That event hasn't happen yet, so it's not a post event layout that includes blog articles.