docs: polish and modernize README

[vdusek]

Summary

The README had fallen behind the rest of the project: a badge-only header, a four-line intro, no table of contents, and no links into the docs site, related projects, or community/contributing sections.

This rewrite follows modern OSS conventions and mirrors the recent apify-client-python README polish (apify/apify-client-python#776), adapted for the SDK:

• Centered tagline and cleaned-up badges (added a License badge, reordered, dropped rel="nofollow").
• Intro describing what the SDK does, plus a callout pointing API-only users to the Apify API client.
• Table of contents.
• Installation via pip and uv, including the scrapy extra.
• Quick start: a minimal Actor plus a CLI scaffold-and-run procedure.
• Features as a linked bullet list pointing into the Concepts and Guides docs.
• Kept the HTTPX + BeautifulSoup and Crawlee Playwright usage examples and the "What are Actors?" section.
• New Documentation table, Related projects, Support and community, Contributing, and License sections.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.00%. Comparing base (a87e8d1) to head (79bffe6).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #958   +/-   ##
=======================================
  Coverage   90.00%   90.00%           
=======================================
  Files          49       49           
  Lines        3053     3053           
=======================================
  Hits         2748     2748           
  Misses        305      305           

Flag	Coverage Δ
e2e	35.66% <ø> (ø)
integration	56.86% <ø> (ø)
unit	78.70% <ø> (ø)

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