Merge pull request #239 from flashcatcloud/docs/rum-sampling-best-practices

docs(rum): add sampling mechanism and best practices guide

Author: Fiona2016

docs.json

@@ -491,6 +491,7 @@
                 "group": "最佳实践",
                 "icon": "lightbulb",
                 "pages": [
+                  "zh/rum/best-practices/sampling",
                   "zh/rum/best-practices/distributed-tracing",
                   "zh/rum/best-practices/alert-noise-reduction"
                 ]
@@ -1755,6 +1756,7 @@
                 "group": "Best Practices",
                 "icon": "lightbulb",
                 "pages": [
+                  "en/rum/best-practices/sampling",
                   "en/rum/best-practices/distributed-tracing",
                   "en/rum/best-practices/alert-noise-reduction"
                 ]