What should be done?

We should have documentation for actions we do and tools we use to check if some application is already compatible with FerretDB and what is missing if it is not. Those tools include:

• manual and automated testing with diff mode;
• manual and automated testing with diff-proxy mode;
• response metrics (that are dumped to stderr on exit in debug build);
• https://github.com/awslabs/amazon-documentdb-tools/tree/master/compat-tool
• something else?

Let's document them in the user documentation as an unlisted page. Then @Fashander could rework it into user-visible documentation as a part of #3229.