Adds FAQ to address common questions regarding Web Install API (#1154)

* Explainer restructuring draft

* Update faq.md

* Update faq.md

* Update faq.md

* Revise Web Install API explainer for background docs

Expanded and clarified the explainer for installing background web apps via the Web Install API. Added new use cases, detailed API signatures, permission model, error handling, and installation flow. Updated references and improved distinction between current and background document installation.

* Update explainer-background-doc.md

* Delete explainer-bd.md

* Update explainer-background-doc.md

* Clarify error handling and update stakeholder section

Refined the explanation of promise rejection errors in the Web Install API documentation, clarifying when specific errors are thrown. Updated references to incognito/private mode and renamed the stakeholder feedback section to include FAQs. Fixed a typo in the FAQ regarding 'abusive' to 'abuse'.

Author: diekus