Combine TOC files to output a single TOC

[JamesNK]

I would like the ability to merge conceptual and API documentation to a single TOC. That is what I'm doing currently with Sandcastle: http://www.newtonsoft.com/json/help/html/Introduction.htm (API documentation expands at the bottom)

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From #148

[danroth27]

We want something similar for ASP.NET. We currently use a hierarchy of sphinx toctree directives to build up our TOC (ex see https://docs.asp.net/en/latest/security/index.html). We want to migrate this structure to a hierarchy of toc.md files that get stitched together.

[danroth27]

@shirhatti

[mscommunities]

We are also looking for the same functionality.

[vicancy]

We introduced TOC reference to support this feature.