Add Child Namespaces

Namespace pages should all include a list of their child namespaces. For example "IO" should be listed under "System". Child namespaces should also have some kind of link to move back up the tree of namespaces -- preferably a bread crumb trail like Windows (File) Explorer provides for folders so it's easy to jump up multiple levels.

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#### Document Details

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* ID: 60c3438c-e2d4-d1d7-88d8-70843d7be3e9
* Version Independent ID: 6b498eb3-40a9-7fb5-3acf-b7e563efcdc6
* Content: [System Namespace](https://docs.microsoft.com/en-us/dotnet/api/system?view=netcore-2.1)
* Content Source: [xml/ns-System.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/ns-System.xml)
* Product: **dotnet-api**
* GitHub Login: @rpetrusha
* Microsoft Alias: **ronpet**