Add section about PostgreSQL dependency

[MatthewVaadin]

Adds a section that describes how to add the PostgreSQL dependency to an application.

Part of https://github.com/vaadin/control-center/issues/964

[github-actions]

AI Language Review

For the file flyway-migrations:

• Improve the phrase "a simple way to set up" to "an easy method for setting up" for better readability.
• In the note section, consider rephrasing “NOTE: The Database feature” to “Note: The database feature” to maintain consistency with the general writing style.

For the file database:

• In the opening sentence, change “a simple way to provision” to “an easy method for provisioning” to enhance clarity.
• The phrase “the PostgreSQL JDBC driver to be included” should be revised to “the PostgreSQL JDBC driver needs to be included” for grammatical accuracy.