docs: add custom HTTP clients concept page and HTTPX guide

[vdusek]

Summary

• Basically documenting Allow to pass own http_client to ApifyClient and ApifyClientAsync #416.

Description

• Add concept page documenting the pluggable HTTP client architecture (default client config, abstract base classes, HttpResponse protocol, call method parameters)
• Add guide page with step-by-step HTTPX implementation example (sync + async)
• Add ApiLink component for linking to API reference from docs

Test plan

• Doc page was rendered and manually checked locally
• CI passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.59%. Comparing base (3fee70a) to head (eefbd1d).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #658      +/-   ##
==========================================
+ Coverage   96.52%   96.59%   +0.06%     
==========================================
  Files          45       45              
  Lines        4318     4318              
==========================================
+ Hits         4168     4171       +3     
+ Misses        150      147       -3     

Flag	Coverage Δ
integration	94.85% <ø> (+0.30%)	⬆️
unit	78.04% <ø> (ø)

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