Automatically add boilerplate summary for auto-added parameterless constructors

[HalfConverter](https://github.com/dotnet/dotnet-api-docs/blob/2dd74450dc9f8f6e7fc586fbcf0f04d34333eaab/xml/System.ComponentModel/HalfConverter.xml?rgh-link-date=2022-08-10T22%3A26%3A39Z#L34), [Int128Converter ](https://github.com/dotnet/dotnet-api-docs/blob/2dd74450dc9f8f6e7fc586fbcf0f04d34333eaab/xml/System.ComponentModel/Int128Converter.xml?rgh-link-date=2022-08-10T22%3A26%3A39Z#L34)or [UInt128Converter](https://github.com/dotnet/dotnet-api-docs/blob/2dd74450dc9f8f6e7fc586fbcf0f04d34333eaab/xml/System.ComponentModel/UInt128Converter.xml?rgh-link-date=2022-08-10T22%3A26%3A39Z#L34), have their documentation xml file created in this location:

runtime\artifacts\bin\System.ComponentModel.TypeConverter\Release\net7.0\System.ComponentModel.TypeConverter.xml

None of these 3 APIs have an xml entry for a parameterless constructor, but it gets added to the dotnet-api-docs file automatically anyway. According to [this programming guide doc](https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/instance-constructors#parameterless-constructors), it's valid.

To fix this:

- We need to agree on a generic boilerplate message that would apply to all APIs. Something like:

> `Creates a new instance with default values.`

- We need investigate if it’s possible to determine if a parameterless constructor was autogenerated by the compiler and that’s why I was unable to find its docs in the intellisense xml files, or if the problem was that the user provided the wrong artifacts path which does not contain the xml file with that type’s docs.

runtime\artifacts\bin\System.ComponentModel.TypeConverter\Release\net7.0\System.ComponentModel.TypeConverter.xml で生成されたエントリを、HalfConverter、Int128Converter、UInt128Converter に対応する dotnet-api-docs の XML ファイルと比較します。パラメーターなしのコンストラクターがコンパイラーによって生成されたものなのか、artifacts path が原因で欠落しているのかを調査し、その後 sync の動作を確認して、汎用的な要約について合意します。原因が確定し、boilerplate の扱いと文言が定義されれば完了です。

索引モデルが issue の本文から書いたものです。