ReadTheDocs versioning

[ewoutkramer]

Add versioning to ReadTheDocs of Firely .NET Docs.

Also try to have the API documentation in ReadTheDocs.

Already done

Make sure you have .NET SDK installed, then open a terminal and enter the following command to install the latest docfx:

dotnet tool update -g docfx

Use the following docfx.json:

{
  "metadata":[ {
    "src": [
      {
        "files": ["**/Hl7.Fhir*.csproj"],
        "exclude": ["**/obj/**", "**/bin/**", "**/BenchMarks/**", "**/Hl7.Fhir*Tests/**"],
        "src": "../src"
      }
    ],
    "dest": "api",
    "properties": {
      "TargetFramework": "netstandard2.0"
  }
  }
  
],
  "build": { 
    "content": [{
      "files": [ "api/*.yml" ]
    }]
  }
}

Place the above json in folder <SDK-repo>\docfx_project

Then run the following command:

docfx docfx_project/docfx.json --serve