Add section on Excel and unicode CSVs and/or consolidate existing info

We currently mention issues with Excel and unicode CSVs in a few places:
* https://github.com/getodk/docs/blob/d5673fb78f3270c1f6465d95eb319c6084e116ca/docs/central-submissions.rst?plain=1#L128
* https://github.com/getodk/docs/blob/d5673fb78f3270c1f6465d95eb319c6084e116ca/docs/aggregate-data-access.rst?plain=1#L33
* https://github.com/getodk/docs/blob/d5673fb78f3270c1f6465d95eb319c6084e116ca/docs/briefcase-using.rst?plain=1#L205

It's relevant to bulk entity creation from CSV as well.

Start by comparing the Excel and Unicode CSV guidance in docs/central-submissions.rst, docs/aggregate-data-access.rst, and docs/briefcase-using.rst. Search the documentation for bulk entity creation from CSV and decide whether the existing information should be consolidated or supplemented. Done means the relevant guidance is complete, consistent, and also covers bulk entity creation.

Written by the indexing model from the issue text.