Add SESSION_COOKIE_SECURE to production docs

[jacobtylerwalls]

brief description of changes

We didn't have explicit guidance around this setting before, but it was warned about by manage.py check --deploy path.to.production.settings, which we recently added to the upgrade instructions in 7.6.0.md for extra visibility, even though we also link to the the django deployment checklist here in this document, which advises the same action).

Do we want to describe the other action items that users are likely going to need to care about after they see the output of manage.py check --deploy?

[aarongundel]

That is a good idea @jacobtylerwalls - if it comes up in the check, we should probably document that or at least link to supporting documentation in django.

[jacobtylerwalls]

Thanks for the review. I'll merge this but will open a follow-up ticket to consider reimagining this page to guide people to manage.py check --deploy

[ekansa]

Thanks! This is very helpful and the security implications are well explained.