docs: update Together README.md (#18004)
## PR message
**Description:** This PR adds a README file for the Together API in the
`libs/partners` folder of this repository. The README includes:
- A brief description of the package
- Installation instructions and class introductions
- Simple usage examples
**Issue:** #17545
This PR only contains document changes.
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Co-authored-by: Bagatur <baskaryan@gmail.com>
