cherry pick from zowe/refactor-troubleshoot-cgw

[Shobhajayanna]

remove central gateway reference from the docs for v3

Describe your pull request here:

remove central gateway reference from the docs for v3

List the file(s) included in this PR:
docs/troubleshoot/troubleshoot-apiml-error-codes.md
docs/user-guide/api-mediation/api-mediation-multi-tenancy.md
docs/user-guide/api-mediation/configuration-multi-tenancy-routing.md
docs/user-guide/api-mediation/diagrams/multi-domain_architecture_V3.svg
docs/whats-new/zowe-v3-frequently-asked-questions.md
versioned_docs/version-v2.18.x/whats-new/zowe-v3-frequently-asked-questions.md

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[github-actions]

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[github-actions]

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