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

[lognaturel]

We currently mention issues with Excel and unicode CSVs in a few places:

• docs/docs/central-submissions.rst

  Line 128 in d5673fb

  Excel will not import CSVs with Unicode characters like ã, ß, and 箸 correctly if you double-click the file or open it from the File menu. You must use the `Text Import Wizard <https://support.microsoft.com/en-us/office/text-import-wizard-c5b02af6-fda1-4440-899f-f78bafe41857>`_ and specify a file origin of Unicode (UTF-8, 65001) and the comma delimiter.

• docs/docs/aggregate-data-access.rst

  Line 33 in d5673fb

  To import CSVs into Excel, you cannot download and open in one step; nor can you double-click on the CSV. You must open Excel and choose Import. If you are asked, the file origin or encoding is UTF-8.

• docs/docs/briefcase-using.rst

  Line 205 in d5673fb

  To import CSVs into Excel, you cannot download and open in one step; nor can you double-click on the CSV. You must open Excel and choose Import. If you are asked, the file origin or encoding is UTF-8.

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