Hi Xiangyang,
The way I would do it is
WolframLanguageData["TimeSeries", "RelatedGuidePages"]
Unfortunately, it evaluates to Missing["NotAvailable"]
. I tried many different symbols and got the same result
AssociationMap[#["RelatedGuidePages"] &, WolframLanguageData["SampleEntities"]]
Looks like a bug. I have reported it on the Wolfram Redmine issue tracker - Bug #3215.