Create a "Custom Layout" user guide topic

[gregvanl]

As pointed out in several recent issues (#6312, #6313, #6326), many advanced editing features are only mentioned in release notes and don't have a place in the main VS Code documentation.

The Get Started > User Interface topic targets new VS Code users and we need to be careful not to overwhelm folks with the vast array of VS Code features, commands, and settings.

We should consider creating a new topic describing advanced features for both completeness and for more advanced users.

[gregvanl]

Topic has morphed from "Advanced Features" to "Custom Layout" as most of the undocumented features had to do with workbench and editor layout customization.