docs: add docs for __FEDERATION__ global window object

[YanPes]

Clear and concise description of the problem

Documentation needed for __FEDERATION__ global window object.
Associated discussion: #2798

Suggested solution

Add docs to the end of "getting started guide"

Alternative

No response

Additional context

No response

Validations

• Read the Contributing Guidelines.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

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