document Mailchimp in guides

[pdurbin]

What this PR does / why we need it:

Which issue(s) this PR closes:

• Closes Update guides to explain use of Mailchimp for security advisories #12443

Special notes for your reviewer:

Suggestions on how to test this:

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

Is there a release notes update needed for this change?:

Additional documentation:

• https://dataverse-guide--12456.org.readthedocs.build/en/12456/installation/config.html#ongoing-security
• https://dataverse-guide--12456.org.readthedocs.build/en/12456/developers/security.html#sending-security-notices

[philippconzett]

I wonder if we should remove the link to the old/"private" spreadsheet. The information is outdated for several installations. At least for DataverseNO, we'd like information only being sent to the email address indicated in the public spreadsheet. My suggestion would be:

Gather email addresses from the .. _public installation spreadsheet ...

[qqmyers]

FWIW: I think that second sheet exists/existed because not everyone wants their security staff publicly identified. That's probably an option we need to keep via some mechanism.

[philippconzett]

OK. Is there a way to remove the DataverseNO record from the "private" spreadsheet?