docs: warn that v4 removes the bundled postgres and document the migration

[dkrizan]

What

Two changes:

1. platform/self_hosting/running_with_docker.mdx — warns that Tolgee v4 removes the bundled PostgreSQL from the tolgee/tolgee image, and adds a Migrate from the bundled database section with the actual procedure.
2. .github/workflows/update-generated-code.yaml — the docs codegen job booted ghcr.io/tolgee/tolgee-ee standalone and relied on its embedded postgres. The EE image is becoming slim (no bundled DB), so the job now starts a postgres service and points the app at it.

Why

In v4 the bundled postgres goes away, so anyone running it has to move to an external database first. The good news is that this needs no dump and restore: the bundled server keeps its files in data/postgres on the volume users already mount, which is exactly where a separate postgres container looks for them. Pin the container to postgres:13 and it adopts the directory in place.

This also fixes a live trap. The existing external-database example runs postgres:15 and mounts ./data/postgres:/var/lib/postgresql/data — the same directory a bundled user already has. Copy-pasting it as a migration fails hard:

FATAL:  database files are incompatible with server
DETAIL: The data directory was initialized by PostgreSQL version 13,
        which is not compatible with this version 15.17

That example is fine for fresh installs and now says it's for fresh installs.

Verified end-to-end

Against the real tolgee/tolgee:v3.212.0 image, not by reasoning:

• Booted with the bundled DB, confirmed PG_VERSION is 13 at data/postgres, created a marker translation key via the API.
• postgres:15 on that directory → exits(1), incompatible-files error above.
• postgres:13 on that directory → starts, all 103 tables intact.
• Tolgee with postgres-autostart.enabled=false against the external postgres:13 → boots, starts no embedded server, original admin password still works, marker key still present.

Notes for review

• "Running Tolgee in single container" keeps a caution for now, deleted when v4 ships.
• Companion PRs: feat: deprecate embedded postgres ahead of v4 tolgee-platform#3610 (startup deprecation warning) · tolgee/billing#274 (EE image switch).

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[JanCizmar]

I would call the documentation sections in this form such as "Keeping ...". Saying "Keep something" sounds like encouraging the user to do such a thing.