Refactors the user docs menus to make them more intuitive: start with tutorials; rename Deployment to How to Run; move Runtime Options under How to Run; rename DynamoRIO Extensions to Extension API and move higher; separate out Building into How to Build a Tool on its own page; misc other changes.
Adds some error checking to the post-build processing so we'll know at build time if a change breaks the web site.
Issue: #4111