feature: add mkdocs-material documentation site with CI deployment

Generate comprehensive documentation for all 173 source units and
integrate mkdocs-material into the build toolchain. Documentation
covers kernel core, x86 architecture, drivers, applications, and
all subsystems with a tabbed navigation structure.

- Add mkdocs.yml with material theme, mermaid support, and tabbed nav
- Add doc/index.md landing page
- Generate doc/src/ markdown for every Pascal unit in src/
- Reorganize planning docs into doc/planning/
- Add compat, lvglh, and toolchain doc sections
- Update Dockerfile to install python3 and mkdocs-material (pinned <2)
- Replace pasdoc-based compile_docs.sh with mkdocs build --strict
- Add compile_docs.sh to the main build pipeline in compile.sh
- Add deploy-docs step in .drone.yml with host volume mount to nginx
- Add site/.gitkeep for build output directory

doc/src/driver/storage/ctl/driver.storage.ctl.usb.md

@@ -0,0 +1,43 @@
+# driver.storage.ctl.usb
+
+USB Mass Storage Class driver stub.
+
+## Overview
+
+This unit registers a USB class driver with the driver manager (`driver.mgr`) to claim USB Mass Storage Class interfaces (class `$08`, subclass `$06` SCSI, protocol `$50` Bulk-Only). When the USB stack enumerates a matching interface, the `load` callback is invoked. Currently the callback only logs a detection message; no I/O infrastructure is established.
+
+Full Bulk-Only Transport (BOT) support — including Max LUN query, endpoint setup, CBW/CSW command exchange, and `driver.storage.mgr` device registration — is planned but not yet implemented.
+
+## Dependencies
+
+- `driver.mgr`
+- `memory.heap`
+- `driver.storage.types`
+- `io.syslog`
+- `debug.tracer`
+- `driver.bus.usb.core`
+- `driver.bus.usb.types`
+- `core.util`, `arch.x86.util`
+
+## Constants
+
+| Constant | Value | Description |
+|---|---|---|
+| `USB_CLASS_MASS_STORAGE` | `$08` | USB interface class for mass storage |
+| `USB_SC_SCSI` | `$06` | SCSI transparent command set subclass |
+| `USB_PROTO_BBB` | `$50` | Bulk-Only (BOT) transport protocol |
+
+## Functions and Procedures
+
+### init
+
+```pascal
+procedure init;
+```
+
+Constructs a `TDeviceIdentifier` matching any USB device with `bInterfaceClass = $08`, `bInterfaceSubClass = $06`, `bInterfaceProtocol = $50`, and registers the `load` callback with `driver.mgr`.
+
+## Notes
+
+- The `load` callback currently returns true unconditionally after logging a detection message.
+- Once full BOT support is implemented, `load` should query the Max LUN, allocate endpoint structures, and call `driver.storage.mgr.register_device` for each logical unit.