Add GitLab Modular Monolith design document

What does this MR do and why?

This merge request is a first step towards a modular monolith architecture. We want to describe what it takes to move from our tightly-coupled monolithic application, to more decoupled modular monolith.

Issue with previous discussions: Proposal: split GitLab monolith into components (#365293 - closed) (thank you @fabiopitino for starting this discussion there).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

/cc @dcroft @sgoldstein