[CB-3827] move plugin spec & plugman doc to new top-level plugin_ref dir
[CB-3827] new 'hybrid' tree contains webview & plugins exposition
[CB-3827] new 'appdev' tree contains whitelist & privacy topics
[CB-3827] move former 'Project Settings' to new top-level config_ref tree
[CB-3827] no more standalone 'Upgrading Guides', so remove this tree