Add documentation for the survey.getStructuredData function

[JaneSjs]

https://github.com/surveyjs/survey-library/blob/9cb2b148b35f92a69c75b958f35ad1e00507b572/src/survey.ts#L2891

User inquiries:

• https://github.com/surveyjs/survey-library/issues/7928
• https://surveyjs.answerdesk.io/internal/ticket/details/T15679
• https://surveyjs.answerdesk.io/internal/ticket/details/T13399
• https://surveyjs.answerdesk.io/internal/ticket/details/T12472
• https://surveyjs.answerdesk.io/internal/ticket/details/T12160