docs: Document the serverDirectory constructor argument

[marcelomendoncasoares]

Summary

The Serverpod constructor takes a serverDirectory, which decides where configuration, passwords, migrations, and relative database paths are resolved from. It was undocumented, so anyone launching the server from somewhere other than the server package root had nothing to go on.

Changes

• Add a "Server directory" subsection to "Configure in Dart" on the configuration page, next to the other constructor arguments.
• Explain the default behavior, that generated projects need no configuration, and when a nonstandard launcher does.
• Show a monorepo launcher pointing at a nested server package.
• List everything resolved from the directory: run-mode config, passwords, migrations including module migrations, and relative SQLite and embedded PostgreSQL paths.
• Note that a ServerpodConfig object still leaves serverDirectory anchoring migrations and relative database paths, and that files your own code opens have to be anchored through pod.serverDirectory.

Reference

• fix: Avoid depending on, or manipulating Directory.current serverpod#5185