[Documentation] Update legacy brand references (Polygon ID) to Privado ID for consistency

[parsavat]

While reviewing the Privado ID Documentation, I noticed several instances where the legacy name "Polygon ID" is used interchangeably with "Privado ID" without the "prev." prefix. To ensure brand consistency and avoid user confusion, these should be updated.

Instances Found:

1. In the FAQ section, specifically under "What is Privado ID?", "How to set up Privado ID?", and "How is blockchain used in Privado ID?".

2. Several paragraphs still use "Polygon ID" as the primary subject.

Proposed Change:
Update all standalone "Polygon ID" mentions to "Privado ID" across the FAQ and tutorial pages.

Additional Note:
I noticed this while verifying my credentials on Billions Network. A consistent naming convention will help new users transition smoothly between the Billions ecosystem and the underlying Privado ID tech stack.