Update dashboard creation guidelines

Context (internal): https://elastic.slack.com/archives/C0A04EAG7N1/p1781774623336209

We now have better design guidelines for creating great dashboards here: https://docs.google.com/document/d/1gRDPy_C1fBWkuEvSzXNY6k6Ii3ubAQseLfXOik7S-KQ/edit?tab=t.0#heading=h.4bnxn2mcm4v0. We should update the https://www.elastic.co/docs/extend/integrations/dashboard-guidelines page at least to update these guidelines, and ideally to reference the official external docs (https://www.elastic.co/docs/explore-analyze/dashboards/arrange-panels#dashboard-grid-layout) for information that exist there to avoid duplication

Start with the dashboard guidelines page at elastic.co/docs/extend/integrations/dashboard-guidelines and compare it with the linked Google Docs guidance and official dashboard grid-layout documentation. Update the guidelines to reflect the newer recommendations and reference the official docs where they already cover the information, with no unnecessary duplication remaining.

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