Add links to external resources

Released in XXXXX

https://sis-cc.gitlab.io/dotstatsuite-documentation/using-de/viewing-data/toolbar/#additional-downloads-of-external-resources

to be complemented (once https://gitlab.com/sis-cc/.stat-suite/dotstatsuite-data-lifecycle-manager/-/issues/161 is delivered)

In the meantime, you can check here the current format supported in SDMX for defining external resources to be downloaded from the DE.