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>

README.md

@@ -25,6 +25,9 @@ Progress and milestone details:
 Latest release (installers for Windows, Linux, and macOS):
 - [v2026.9.8.2](https://git.darksingularity.org/DarkSingularity/orbithub/releases/tag/v2026.9.8.2)
 
+User Guide:
+- [docs/USER_GUIDE.md](docs/USER_GUIDE.md) (also available as a PDF attached to each release, and in-app via `Help -> User Guide`)
+
 ## Screenshots
 
 ![Profile list with folders](docs/images/screenshot-profiles.png)