[WIP] Add overview page for Federated Gateway documentation

[Copilot]

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Missing Overveiw page for Federated Gateway documentation</issue_title>
<issue_description>### Description

Currently, the API Manager documentation doesn't have an Overview page for the Federated gateway document

Steps to Reproduce

N/A

Version

4.6.0

Environment Details (with versions)

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #4454

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