docs(plc): add PLC Runtime Landscape reference + support watch-list

Documents the soft-PLC runtime ecosystem as a support watch-list: we support the
CODESYS family today (incl. the many rebranded-CODESYS OEMs — Schneider, WAGO,
ABB, Bosch, Eaton, KEBA), and everything else (ProConOS/ISaGRAF/straton, Siemens
TIA, Rockwell, Beckhoff TwinCAT, B&R, Mitsubishi, Omron, PLCnext) is watch-list —
we add a project-format parser when a customer needs it. The dynamic OT probe is
vendor-agnostic. Also lists the open-source test-bench substrates (OpenPLC,
Beremiz+MatIEC, 4diac). New Reference sidebar entry.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/.vitepress/config.mts

@@ -44,6 +44,7 @@ export default withMermaid(defineConfig({
         items: [
           { text: 'Glossary', link: '/reference/glossary' },
           { text: 'Tools & Scanners', link: '/reference/tools' },
+          { text: 'PLC Runtime Landscape', link: '/reference/plc-runtimes' },
         ],
       },
     ],