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>

tools/user-guide-pdf/CMakeLists.txt

@@ -0,0 +1,11 @@
+cmake_minimum_required(VERSION 3.21)
+
+project(OrbitHubUserGuidePdf LANGUAGES CXX)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+find_package(Qt6 6.2 REQUIRED COMPONENTS Widgets PrintSupport)
+
+add_executable(user-guide-pdf main.cpp)
+target_link_libraries(user-guide-pdf PRIVATE Qt6::Widgets Qt6::PrintSupport)