Docs: distinguish domain code from generic code

What does this MR do and why?

• Related to Clarify where to place code in lib (#412652 - closed)

In this MR we update the software design guides to describe how to separate generic code from domain code, linking also to https://docs.gitlab.com/ee/development/gems for the guidelines on how to do that.

• Domain code is unique to GitLab and describe the business logic, policies and data.
  • We currently keep this were it is, as long as it's correctly namespaced.
• Generic code can be used in other projects, could be rewritten or could be replaced with off-the-shelf solution.
  • We gradually move this from lib/ to gems/.

MR acceptance checklist

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