Add a design document for merging CE and EE codebases

Opening a discussion about making the decision on what steps we can take towards merging CE and EE codebases

Review app direct link: https://mj-design-ce-ee-codebases-merge.about-src.gitlab.com/handbook/engineering/infrastructure/blueprint/ce-ee-codebases/index.html

https://mj-design-ce-ee-codebases-merge.about-src.gitlab.com/handbook/engineering/infrastructure/design/201901_Merging_CE_EE_codebases.html

/cc @gitlab-org/delivery