Document that the XML schema has been fixed and how to use it

[dean-krueger]

PR 1846 from Cyclus/Cyclus fixed the XML schema grammar and allows the VS Code XML plugin by Red Hat to become a very powerful tool for generating input files (especially for new users). This feature should be documented on the website (likely as a part of this tutorial). Specifically:

• Document that the schema grammar is a really powerful tool for generating input files and recommend/explain its use
• Document which VS Code plugins need to be installed to enable this tool