Security: Generate and document GPG key(s)

Has this feature been discussed and generally agreed?

jamulussoftware/jamuluswebsite#319 was about documenting the process and a contact address for reporting security issues. It was also discussed to provide a way to contact the project in encrypted form. This still needs to be done.

I'm opening this on the code repo as this is where SECURITY.md lives and will have to be updated.

Describe the solution you'd like

Variants:

1. Document each developer's own GPG key (and address) on their team page (tbd)
2. Generate a key for team@ and share it.
   Both variants have advantages and disadvantages, see https://github.com/jamulussoftware/jamuluswebsite/issues/319#issuecomment-803663521 and later comments.