[paperles] Set PAPERLES_URL automatically

[angelnu]

Description of the change

Set PAPERLESS_URL by default to the main ingress host - see https://github.com/paperless-ngx/paperless-ngx/issues/712

Benefits

Working helm chart after upgrading to paperless-ngx >=v1.7

Possible drawbacks

None

Applicable issues

• fixes #1538

Additional information

Checklist

• [x] Title of the PR starts with chart name (e.g. [home-assistant])
• [x] Chart version bumped in Chart.yaml according to semver.
• [x] Chart artifacthub.io/changes changelog annotation has been updated in Chart.yaml. See Artifact Hub documentation for more info.
• [x] Variables have been documented in the values.yaml file.