Document the recommended CiviMail permissions for anonymous users

[wmortada]

The page on Permissions and access control is a really helpful resource. I've just been referring to it myself to check the permissions given to users.

However, I think it is missing some documentation on the default CiviMail permissions for anonymous users. Namely for:

• view_public_civimail_content
• access_civimail_subscribe_unsubscribe_pages

I think it would also be helpful to provide a reference to this on the page for setting up CiviMail.

[github-actions]

This issue has had no activity in 60 days and has been marked as stale, it will be closed in 10 days.

[wmortada]

This still appears to need improvement. I'll take a look at this.

[wmortada]

As a step towards this I'm attempting to clarify what the initial permissions for anonymous users are in #441.