Create article: Routing requests to REST APIs

[janan07]

Describe your pull request here: Create the article Routing requests to REST APIs

List the file(s) included in this PR: docs/user-guide/api-mediation/routing-requests-to-rest-apis.md

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[github-actions]

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[balhar-jakub]

I changed some texts and there is one note from me, but in general it looks well.

[github-actions]

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