Added Version number to mkdocs.yml (#445)

## Fixes
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## Changes
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## Checklist
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- [ ] 🤖 This change is covered by unit tests (if applicable).
- [ ] 🤹 Manual testing has been performed (if necessary).
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- [ ] 📖 Documentation updates are complete (if required).
- [ ] 🧠 Third-party dependencies and TPIP updated (if required).

Author: ReinhardKeil

mkdocs.yml

@@ -1,4 +1,6 @@
 site_name: CMSIS-Toolbox
+extra:
+  version: 2.12.0
 repo_url: https://github.com/Open-CMSIS-Pack/cmsis-toolbox
 edit_uri: ./edit/main/docs
 nav: