Tracing: Add README files for tracing datasource plugins

public/app/plugins/datasource/jaeger/README.md

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+# Jaeger Data Source - Native Plugin
+
+Grafana ships with **built in** support for Jaeger, an open source, end-to-end distributed tracing system.
+
+Read more about it here:
+
+[https://docs.grafana.org/datasources/jaeger/](https://docs.grafana.org/datasources/jaeger/)

public/app/plugins/datasource/tempo/README.md

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+# Tempo Data Source - Native Plugin
+
+Grafana ships with **built in** support for Tempo, an open source, easy-to-use, and high-scale distributed tracing backend.
+
+Read more about it here:
+
+[https://docs.grafana.org/datasources/tempo/](https://docs.grafana.org/datasources/tempo/)

public/app/plugins/datasource/zipkin/README.md

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+# Zipkin Data Source - Native Plugin
+
+Grafana ships with **built in** support for Zipkin, an open source, distributed tracing system.
+
+Read more about it here:
+
+[https://docs.grafana.org/datasources/zipkin/](https://docs.grafana.org/datasources/zipkin/)