How can we achieve something like password protected anonymous access? We don't want our documentation to be public, only our customers should have access to it without having a Confluence user account.
Sharing a single user for all of your customers would infringe the Confluence license agreement. There are different solutions to password-protect your Confluence content for Anonymous users:
Apache HTTP server and Confluence
Use an Apache HTTP Server with the mod_proxy extension to enable basic http authentication for certain URLs. This way you could open up your documentation space for anonymous access and protect this URL using Apache. You can than decide to configure credentials for each user or customer, or a single one for all of them.
Using an anonymous user kind of restricts you from using the collaboration features of Confluence such as share, @mention, or most notably, comments.
Apache HTTP server and Scroll HTML Exporter
In this solution you would publish your documentation using the Scroll HTML Exporter and making it available using the Apache server.