Restructure extending the API Mediation Layer

[balhar-jakub]

Describe your pull request here:
Finish the restructuring of the existing Extend documentation.

List the file(s) included in this PR: sidebar.js

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[github-actions]

😺 Thank you for creating this PR! To publish your content to Zowe Docs, follow these required steps.

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[github-actions]

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[github-actions]

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[janan07]

Reviewed by doc squad