Add banner linking to Grafana docs

[heitortsergent]

What?

Add a banner to all the pages linking to the Grafana docs.

Checklist

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[heitortsergent]

@ppcano I remembered we have the banner feature, similar to what we used for the forum migration.

What do you think if we use this for the docs as well? :)

[ppcano]

@heitortsergent nice catch, LGTM!