Improve manpage infrastucture

The man pages for various commands (such as dotnet-build) indicate they are generated from some other document using a tool:

$ head -1 ./dotnet-sdk-2.0.0-preview2-006497/src/cli/Documentation/manpages/sdk/dotnet-compile.1
.\" Automatically generated by Pandoc 1.15.1

What is the process to update these docs? Where can I find that original source?

[ghost]

@nguerrera, @richlander, @Petermarcu, if hooking up pandoc script in CI is not possible for 2.1 (https://github.com/dotnet/docs/issues/2791#issuecomment-372707046), could we run pandoc script locally and send PR somewhere with all converted manpages for review? Maybe the auto-converted md files will require some manual changes. It would be very helpful if 2.1 ships out with new manpages. Thanks!

[omajid]

Also, it looks like what we want man pages to look like is already maintained here: https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet-publish. Sources are https://github.com/dotnet/docs/tree/master/docs/core/tools

These documents are almost already in the format of man pages. Even the section names pretty much match up. Maybe it would be better to use those (somehow) rather than duplicating them. As we can already see, those docs are pretty well maintained while these man pages are completely out of date.