Allow the README.MD file of a private project to be publicly viewed

Problem to solve

Guests on private projects cannot view the readme.md file of a project. They are instead redirected to the wiki or issues pages as a fallback while other users with repository access do view the readme.md file

Further details

For private projects that do not need to use an external home page, I think it would be useful to have a single home page for all kinds of users. I think that's the goal of the readme.md file. Overall this will concern people that have small projects they want to keep private (keep the source code hidden from guest users) but still want to use the gitlab project page as an entry point for guest users

Proposal

Add an option to set the readme.md file as the main home page for all users, including guest users on a private project

Permissions and Security

This should be an option set by the maintainer of the project. Security wise, maybe the readme.md file should be extracted somehow as the original problem this feature proposal tries to solve is caused by the fact that the repository of a private project is not visible by guest users

Documentation

This would impact changes proposed in #18739 (closed)

Testing

Test that this solution doesn't compromise the permission settings of a guest user on a private project and that the visibility of the readme.md file is clearly understood by the maintainer changing the setting for that file

What does success look like, and how can we measure that?

If this feature is working as intended, check the number of private projects with guest accounts that are enabling this feature.

What is the type of buyer?

I think this is a feature that would be used by projects that do not have an external home page for their product (meaning small private projects) so I would argue it should be for the free version of gitlab