docs: cleanup readme; add python 3.12 (#3120)
### Summary
Updates documentation references in the README to point to
https://docs.unstructured.io and cleans up a few sections of the README.
Specifically:
- Removes an old API announcement
- Removes the section mentioning Chipper as a beta feature. Chipper is
only available through the SaaS API.
Also adds a Python 3.12 tag to `setup.py` since we now support Python
3.12.
