docs: polish and modernize README (#776)

## Summary

The README was outdated (Quick start used a placeholder Actor, Features
section was a four-bullet wall of text, no TOC, no links to the
documentation site, no related-projects or community sections). This PR
rewrites it to follow modern OSS conventions and to act as a proper
entry point into the docs at
https://docs.apify.com/api/client/python/docs.