There are many aspects of PostgreSQL which are not solved problems e.g. clustering, replication, high-availability. And will remain unsolved whilst they are not part of the core products. Because everyone comes up with different solutions which change over time.
Citus Data which people often recommended didn't exist prior to 2010 and may not exist next year now they've been acquired by Microsoft and conflict with their Azure business.
Clustering, replication and high availability may not be easy, but there are thousands of production PostgreSQL instances out there have solved them. That's what makes it boring.
Compare solving those problems to solving them with a brand new database engine where you might be one of the first organizations to run at that scale.
OK, you've attracted my interest. What are some examples of official database documentation pages that do a great job of covering these advanced deployment topics?
It's important to note that "boring technology" doesn't mean the official documentation will answer all of your questions: it means that there is enough experience out there in the user community that you can find the information if you go looking for it.
How is it an "advanced" topic? Having multiple instances so that you don't have a SPOF is table stakes for any service you care about, and something any half-serious datastore like Cassandra, Kafka or even MongoDB (for all of which "clustering, replication and high availability" are boring things that you would naturally set up without making a fuss about it) will do out of the box. I'm constantly baffled that when it comes to RDBMSes people suddenly decide that single points of failure are fine now actually.
Citus Data which people often recommended didn't exist prior to 2010 and may not exist next year now they've been acquired by Microsoft and conflict with their Azure business.