drivers: adc: ad9680: Add README documentation for AD9680

[AndyOesmer]

Added the readme file for the AD9680 driver.

Modified the sphinx drivers_doc.rst file to add the corresponding source.

PR Type

• [ ] Bug fix (change that fixes an issue)
• [ ] New feature (change that adds new functionality)
• [ ] Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• [ ] I have followed the Coding style guidelines
• [ ] I have complied with the Submission Checklist
• [ ] I have performed a self-review of the changes
• [ ] I have commented my code, at least hard-to-understand parts
• [ ] I have build all projects affected by the changes in this PR
• [ ] I have tested in hardware affected projects, at the relevant boards
• [X] I have signed off all commits from this PR
• [ ] I have updated the documentation (wiki pages, ReadMe etc), if applies

[CLAassistant]

All committers have signed the CLA.

[buha]

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