C# Xml doc (#798)

* add xml documentation TOC

* add description of XML doc tags

* add author name

* add brief descriptions for each XML tag

* update description of XML documentation tags

* add code samples for some tags

* complete tag samples

* semantically arrange tags

* update toc

* address PR feedback

* address PR feedback

* address PR comments

* address PR feedback

* complete new draft based on PR feedback

* fix typos

* address PR  comments

* address PR comments

* address PR comments