docs: Document embedded PostgreSQL

[marcelomendoncasoares]

Summary

Embedded PostgreSQL is the default for the development and test run modes in new projects, and the configuration page introduces dataPath, but nothing explained what the setting actually does, where the files live, or when to move off it. This fills that in on the database connection page.

Changes

• Replace "Development database" with an "Embedded PostgreSQL" section and move the Docker instructions into their own section.
• Explain what dataPath starts, and link to the configuration page rather than repeating it.
• Cover storage: relative path resolution, one directory per run mode, keeping it out of version control, the first-run binary download and its per-user cache location, and the supported platforms.
• Cover lifecycle: startup before the connection pool, the Unix socket transport, attaching to a cluster another process already runs, persistence across restarts, and recovery after an unclean exit.
• Document resetting the database by deleting dataPath, with a warning about the data loss.
• State the production boundary and how to move to an externally managed instance.
• Add an advanced section for using serverpod_embedded_postgres directly: start, transports, connection details, attach, prefetch, stop, reset, and the sealed exception type.
• Update the SERVERPOD_DATABASE_DATA_PATH row in the configuration reference.

Reference

• feat: Introduce serverpod_embedded_postgres package serverpod#5090, fix: Recover orphaned embedded Postgres on Windows after unclean exit serverpod#5316
• Create a dart package for installing and running postgres locally serverpod#5087, ☂️ Allow the using Postgres locally without docker for development serverpod#5068, Improve lifetime management of DatabasePoolConnection. serverpod#2421