ETR01SDK-568: Add STM32U5xx HAL documentation

[medexs]

Description

Adds documentatio for the STM32U5xx HAL.

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Type of Change

Select the type(s) that best describe your change:

• 🐛 Bug fix
• ✨ New feature
• 🧹 Code cleanup or refactoring
• 📝 Documentation update
• 🔧 Build system or toolchain update
• 🔒 Security improvement
• Other (please describe):

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Checklist

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• I opened the Pull Request to the develop branch
• I followed the project's code guidelines
• I formatted the code using clang-format with the recommended configuration
• I updated the changelog, or this change does not require it (e.g., internal or non-functional update)
• The project builds without errors or warnings
• I have verified the functionality against the hardware/model as applicable
• I have ensured that public APIs remain backward compatible (if applicable)
• This PR is ready for review by maintainers (no WIP commits left) and marked as Ready for Review

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[andreondra]

An idea: maybe we can note the specific model that we tested the STM32U5xx HAL on? This will serve as a disclaimer when something goes wrong + maybe as a tip for users which model to buy to be 100% sure it'll work?