[Docs] [Enhancement Request] Add some more information details to the config of exclude

**Is your feature request related to a problem? Please describe.**

Reading through the docs of docfx > Config page, I am missing information about why we might consider excluding the includes path. Dont get me wrong, I did see it on multiple docfx created repos already, where this folder did include `[!INCLUDE ` files.

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would be nice, if there would be some informations about how this is related to the used markdown and what would be the result when not excluding those. same seen for the `_site/**` and `obj/**` ( in overwrite part of the docfx.json build, but I would not know why we should do this.

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Keep guessing and fiddling for hours or days currently to finally get the docfx setup to the desired result of output.

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page of the docs:
https://dotnet.github.io/docfx/docs/config.html first topic