Cross link back to upgrade-landscape page to differentiate charm refresh with package-upgrade

In Step 4 of "[How to migrate to the 26.04 charm](https://ubuntu.com/landscape/docs/how-to-guides/upgrade/migrate-to-26-04-charm/#step-4-refresh-the-charm)" says the PPA must be updated manually on each unit, but there is no link to the detailed procedure. The detailed procedure is at https://ubuntu.com/landscape/docs/how-to-guides/upgrade/upgrade-landscape/

We should have a link back to the "upgrade-landscape" page to clarify the process.

Open Step 4 of the “How to migrate to the 26.04 charm” page and compare it with the detailed procedure in the linked “upgrade-landscape” page. Add a cross-link that distinguishes the charm refresh from the package upgrade, then verify both documentation links resolve and the process is clear.

Written by the indexing model from the issue text.