docs/playgrounds: add aliases for old links

The old links were removed in https://github.com/VictoriaMetrics/VictoriaMetrics/pull/10754
mistakenly thinking that google didn't index it. However, it did. And users can get 404
when searching in google for VM plyagrounds.

Restoring the links via aliases. It means hugo will serve the `/playgrounds` page when
user requests `/playgrounds/victoriametrics/`.

Signed-off-by: hagen1778 <roman@victoriametrics.com>

docs/playgrounds/_index.md

@@ -10,5 +10,11 @@ tags:
   - logs
   - traces
   - playground
+aliases:
+  - /playgrounds/victoriametrics/
+  - /playgrounds/victorialogs/
+  - /playgrounds/victoriatraces/
+  - /playgrounds/cloud/
+  - /playgrounds/vmanomaly/
 ---
 {{% content "README.md" %}}