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 89.93%. Comparing base (a87e8d1) to head (cc1ff05).
⚠️ Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #958      +/-   ##
==========================================
- Coverage   90.00%   89.93%   -0.08%     
==========================================
  Files          49       49              
  Lines        3053     3090      +37     
==========================================
+ Hits         2748     2779      +31     
- Misses        305      311       +6     

Flag	Coverage Δ
e2e	35.92% <ø> (+0.25%)	⬆️
integration	56.92% <ø> (+0.06%)	⬆️
unit	78.73% <ø> (+0.02%)	⬆️

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