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

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