Geo: Add docs on the self-service framework

This is an initial proposal for #196853 (closed). There's a lot of room for improvement, but it might be subject to change at any point now.

• Move Self-Service section into its own doc like doc/development/geo/replicate_a_new_data_type.md
• Link to it from https://gitlab.com/gitlab-org/gitlab/blob/v12.6.3-ee/doc/development/geo.md#steps-needed-to-replicate-a-new-data-type
• Add caveat about it being alpha or under development