docs: document slim Docker image and run codegen against external postgres

[dkrizan]

What

Two slim-image-related changes:

1. platform/self_hosting/running_with_docker.mdx — documents the new public tolgee/tolgee:slim image so self-hosters can actually use it. Adds a Running the slim image section (when to use it, tags, an external-DB compose example) and a pointer from the intro note.
2. .github/workflows/update-generated-code.yaml — the docs codegen job booted ghcr.io/tolgee/tolgee-ee standalone and relied on its embedded postgres. Since the EE image is becoming slim (no bundled DB), the job now starts a postgres service and points the app at it via --network host + SPRING_DATASOURCE_*.

Why

• The slim image ships in feat: add slim Docker image variant without embedded postgres tolgee-platform#3610 but nothing told users it exists or that it needs an external DB.
• Once the EE image is slim (companion billing PR), the codegen workflow would otherwise fail to boot without a database.

Depends on

• feat: add slim Docker image variant without embedded postgres tolgee-platform#3610 — ships the public tolgee/tolgee:slim image the doc describes.
• tolgee/billing PR (companion) — switches the EE image to slim; the codegen change here keeps that workflow green.

Draft until #3610 lands.

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