docs: add link to serverless api in readme (#3322)
### Summary
Adds links to the serverless api. README updates look like the
following:
<img width="904" alt="image"
src="https://github.com/Unstructured-IO/unstructured/assets/1635179/fcb2b0c5-0dff-4612-8f18-62836ca6de8b">
