GitLab Gems: Add documentation about development guidelines

What does this MR do and why?

Related to:

• https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/2926+
• Add `gitlab-rspec` gem (!121676 - merged)
• Introduce `gems/gitlab-utils` gem (!121526 - merged)

The purpose of this change is to bring clarity and structure to monorepo Gems. The usage of monorepo Gems will improve readability of the codebase and be a desired place to store all aux code that is not directly related on business logic of GitLab.

Using monorepo is HIGHLY desired to avoid overhead of using maintaing and publishing Gems via other repository.

This also clarifies the usage of the vendor/gems/.

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.