Explicitly require public visibility of emails

What does this MR do?

The GitHub import performs a REST API call to fetch a user's email to associate their content to their GitLab account. The REST API of GitHub will show a null email address field if the primary email address is not marked to be publicly visible in the account's profile settings page.

Therefore the doc must (more) explicitly state the need for a publicly visible email address on the GitHub end prior to the import.

See also: https://docs.github.com/en/free-pro-team@latest/rest/reference/users#get-a-user

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team