docs: Improve and modernize README

[vdusek]

Polishes and modernizes the README, mirroring the recent apify-client-python polish (apify/apify-client-python#776).

• Reworked intro and badge row (added a CI build-status badge), plus a table of contents.
• Reordered sections, with a new What are Actors? up front and a What you can build section (scraping, browser automation, AI agents, MCP servers, web servers).
• Added Documentation, Related projects, Support and community, and Contributing sections.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.00%. Comparing base (a87e8d1) to head (48d7273).
⚠️ 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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