Groupby / ixref tutorial enhancement

[dxtrous]

Is your feature request related to a problem? Please describe.
The examples in the tutorial at https://pathway.com/developers/user-guide/data-transformation/indexing-grouped-tables don't mention the name / identifier of the employee in any column in the source data. This makes it harder to tell apart groupby results (aggregates) from source data.

Describe the solution you'd like
Update the tutorial, e.g., add first names of employees as an extra column in input example tables (Alice, Bob, Charlie, etc.). Leaving decisions / priorities to @olruas.

Describe alternatives you've considered
n/a.

Additional context
This could also make the tutorial more immediately relatable when comparing to SQL-windowing syntax like in #19.