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/app/app.meminfo.md

@@ -0,0 +1,41 @@
+# app.meminfo
+
+Terminal command for displaying physical and heap memory statistics.
+
+## Overview
+
+`app.meminfo` registers the `MEMINFO` shell command, which prints a summary of the system's memory state in three sections: Multiboot-reported memory ranges, Physical Memory Manager (PMM) block statistics, and Logical Memory Manager (LMM) heap statistics.
+
+## Dependencies
+
+- `io.stdio`
+- `arch.x86.multiboot`
+- `arch.x86.memory.physical`
+- `memory.heap`
+- `debug.tracer`
+
+## Functions and Procedures
+
+### init
+
+```pascal
+procedure init();
+```
+
+Registers the `MEMINFO` command with `io.stdio`.
+
+### run (internal)
+
+```pascal
+procedure run(Params: PParamList; stdin_buf, stdout_buf, stderr_buf: POutBuf);
+```
+
+Command entry point. Outputs three labelled sections:
+
+**Multiboot:** Reads `multibootinfo^.mem_lower` and `mem_upper` to report lower memory, upper memory, and total memory in KB/MB.
+
+**PMM (4 MB blocks):** Reads `arch.x86.memory.physical.pmm_total_blocks`, `pmm_free_blocks`, and derives used blocks. Reports free physical memory in MB.
+
+**LMM (Heap):** Reads `memory.heap.lmm_page_count` and `lmm_total_free`. Reports heap size in MB and free heap in KB.
+
+All output is written to `stdout_buf`.