Add an in-app User Guide and standalone PDF

Adds docs/USER_GUIDE.md, a 10-section end-user guide (getting
started, managing/organizing profiles, SSH and RDP connections,
session management, settings, troubleshooting). It's embedded into
the app binary via a Qt resource file and rendered by a new
Help -> User Guide window: a topic sidebar plus content pane, not a
single scrolling document, with cross-reference links between
sections routed to sidebar selection rather than relying on Qt's
Markdown importer's lack of heading anchors.

A separate, non-shipped tool (tools/user-guide-pdf/) renders the
same source to a standalone PDF via QTextDocument + QPrinter,
wrapped by packaging/docs/build-user-guide-pdf.sh. Kept fully
outside the main CMake target so Qt6::PrintSupport never becomes a
runtime dependency of the shipped app (confirmed via ldd). The PDF
itself isn't committed -- generated per release like the platform
installers.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/session_window.cpp

@@ -2,6 +2,7 @@
 
 #include "about_dialog.h"
 #include "profiles_window.h"
+#include "user_guide_dialog.h"
 #include <QApplication>
 #include "session_tab.h"
 
@@ -182,6 +183,14 @@ SessionWindow::SessionWindow(QWidget* parent)
     connect(quitAction, &QAction::triggered, this, []() { qApp->quit(); });
 
     QMenu* helpMenu = menuBar()->addMenu(QStringLiteral("Help"));
+    QAction* userGuideAction = helpMenu->addAction(QStringLiteral("User Guide"));
+    connect(userGuideAction,
+            &QAction::triggered,
+            this,
+            [this]() {
+                UserGuideDialog dialog(this);
+                dialog.exec();
+            });
     QAction* aboutAction = helpMenu->addAction(QStringLiteral("About OrbitHub"));
     connect(aboutAction,
             &QAction::triggered,