Update docs for keyword separator

[colinldbd]

I found a potential need for improvement in the documentation for JabRef while trying to solve issue#177

In the documenation for JabRef the keyword separator TextField belong to the preference->Group:

However, in the latest implementation, it's no longer there, now it's moved to preference->Entry:

I suggest that we should update the documentation in docs.jabref.org for this to avoid confusion.

[Siedlerchr]

Freel free to create a PR https://github.com/JabRef/user-documentation/blob/main/en/finding-sorting-and-cleaning-entries/groups.md

[Daniel-Ruan]

My teammates and I from ANU would like to take on the task of updating the documentation for the keyword separator as outlined in issue #468. We believe we can contribute effectively to resolving this. Could you please assign this issue to us?

[lluukii]

I am a student in the Australian National University, and I am interested in working on this issue. Could you please assign this issue to me?