Remove outputting md files to AppVeyor artifacts (#16736)
Previously, markdown files were built as an intermittent step between converting text2tags content to HTML.
During this period, it was useful for builds to upload their markdown files as AppVeyor artifacts in the builds system.
We have now removed the t2t content in favour of a markdown source of truth, meaning that the artifact uploaded should be the same as the contents in the PR.
Description of user facing changes
Remove userGuide and changes markdown files from build artifacts
Description of development approach
Remove userGuide and changes markdown files from build artifacts
Also removed other deprecated code and updated remaining references to t2t
