drivers: adc: ad9434: Add README doc for AD9434

[jvelasc2]

Added the rst readme document for the AD9434 driver.

Also modified the drivers_doc.rst to add the corresponding source for this new readme.

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
• [x] I have complied with the Submission Checklist
• [x] 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

[jvelasc2]

Hi @buha , I have already updated this PR following the new sphinx source folder categories. Thank you!