Link Bitnami PostgreSQL docs

What does this MR do?

Link Bitnami PostgreSQL docs

Refer to the PostgreSQL chart documentation instead of
telling users to inspect the default values via helm.

The inspect was a workaround because the PstgreSQL chart docs imported into the GitLab <7.0 chart were not easily accessible.

Now that we are using a current version, we can link to the official docs.

Related issues

Relates #3463 (closed)

Relates #4118 (closed)

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