docs: what's new in v5.3 placeholder

docs/sources/guides/whats-new-in-v5-3.md

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+title = "What's New in Grafana v5.3"
+description = "Feature & improvement highlights for Grafana v5.3"
+keywords = ["grafana", "new", "documentation", "5.3"]
+type = "docs"
+[menu.docs]
+name = "Version 5.3"
+identifier = "v5.3"
+parent = "whatsnew"
+weight = -9
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+
+# What's New in Grafana v5.3
+
+## Changelog
+
+Checkout the [CHANGELOG.md](https://github.com/grafana/grafana/blob/master/CHANGELOG.md) file for a complete list
+of new features, changes, and bug fixes.