Update project page for PostgreSQL service

[DataTideHH]

What changed

• add the local PostgreSQL 18 Docker Compose service to the project page
• clarify that H2 remains the active application and test database
• add PostgreSQL start and stop commands
• correct the sample-data description to refer to integration-test fixtures
• update the documented limitations and next persistence steps

Scope

This pull request only updates docs/index.md. It does not change application configuration or runtime behavior.

Validation

• project page checked against the current README and compose.yaml
• no Java, Maven or application files changed

[Copilot]

Pull request overview

Updates the project’s documentation landing page to reflect the presence of a local PostgreSQL 18 Docker Compose service while clarifying that the application and tests still run against H2.

Changes:

• Adds PostgreSQL Docker Compose service guidance (start/stop) and clarifies it’s not yet wired into the Spring Boot app.
• Clarifies H2 remains the active database for runtime and tests.
• Updates the “Data and limitations” section to describe integration-test fixtures and next persistence steps.

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