doc: Better to update the system before registering the client against Landscape Server

https://github.com/canonical/landscape-documentation/blob/8d5bd31e7cb2bc8fd408b6e6dfa25b7a61c7013b/docs/how-to-guides/landscape-installation-and-set-up/install-landscape-client.md?plain=1#L10-L14

There are some limitations that it takes time to synchronize installed/available package information when the system is not up to date. To maximize the smoothness of the initial Landscape experience, it would be good to make sure to run `apt upgrade` for example.

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Document: getting-started-with-landscape.md

Open the linked install-landscape-client.md section at lines 10–14 and locate the referenced getting-started-with-landscape.md document. Confirm where the initial client setup guidance belongs, then ensure the document explains that updating the system before client registration avoids delayed package synchronization; done means the recommendation is present in the appropriate setup step.

Written by the indexing model from the issue text.