Restructure routing explanation

[balhar-jakub]

Describe your pull request here:
As part of ongoing improvements for the Extend documentation for the API Mediation Layer, improve the structure and add diagrams to make the understanding of how the routing works, easier.

List the file(s) included in this PR: api-mediation-routing.md

After creating the PR, follow the instructions in the comments.

[github-actions]

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[github-actions]

🚀 Deployed on https://66507641c8ae7d2251ea45a2--zowe-docs-master.netlify.app

[balhar-jakub]

@janan07 I answered all the questions in the article, what are the next steps?

[janan07]

Reviewed and edited by Doc squad