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>

docs/BUILDING.md

@@ -140,3 +140,19 @@ right-click → **Open** to bypass Gatekeeper's unidentified-developer warning.
 
 Output path:
 - `dist/macos/OrbitHub-<version>.dmg`
+
+## User Guide PDF
+
+The in-app User Guide (`Help -> User Guide`) is built from
+`docs/USER_GUIDE.md` and embedded into the app at compile time — no extra
+step needed for that. A standalone PDF version is generated separately by a
+small tool (kept out of the main app's dependencies, since it needs
+`Qt6::PrintSupport`):
+
+```bash
+./packaging/docs/build-user-guide-pdf.sh
+```
+
+Output path:
+- `docs/USER_GUIDE.pdf` (not committed to git — a release asset, like the
+  platform installers)