Describe reduced paradata set in paradata format article

[radyakin]

To document surveysolutions/surveysolutions#3729

Here: https://docs.mysurvey.solutions/headquarters/export/paradata_file_format/
add the following paragraph:

Reduced paradata set

Reduced paradata set does not include events with the following codes:

• QuestionEnabled (13),
• QuestionDisabled (14),
• GroupEnabled (15),
• GroupDisabled (16),
• QuestionDeclaredValid (17),
• QuestionDeclaredInvalid (18),
• VariableEnabled (23), and
• VariableDisabled (24).

These events are included into the (default) export of all paradata events.
The events in the reduced paradata file are numbered sequentially from 1 onwards, and because some events are omitted from this file the numbering will generally be different from the numbering of the same events in the full paradata file.