`element.io/en/interoperable-digital-sovereignty` should link to `element.io/en/matrix-in-europe`

At the bottom of https://element.io/en/interoperable-digital-sovereignty, there is a section on "Vendor-agnostic federation" that shows a map of Europe with all of Matrix deployments. We should link to https://element.io/en/matrix-in-europe which gives an interactive exploration with more info.

Open the interoperable digital sovereignty page and locate the “Vendor-agnostic federation” section near the bottom. Add a link to the Matrix in Europe page, then verify that it appears correctly and points to the requested URL.

Written by the indexing model from the issue text.