Manual:$wgWhitelistRead

Details
If a group of users is blocked from viewing the wiki by using the  setting...

...you may still want them to be able to view certain key pages, in particular the login page!

This setting holds an array of page names that all users are allowed to view, regardless of their group permissions.

A recommended minimum which allows everyone to view the home page and the login screen as well as loading CSS/JS customizations is as follows:

"Main Page" uses spaces instead of underscores between words.

If you are using a content language other than English, you may need to use the translated special page names instead of their English names.