docs: Rewrite the custom HTTP client guide around the transport-hook contract

[vdusek]

Rewrites the HTTP clients concepts page around the three-layer hierarchy and the transport hooks, and replaces the HTTPX guide with "Build a custom HTTP client", which implements full AIOHTTP and requests clients including response adapters for the HttpResponse protocol.

Split out of #1006. Stacked on #1012.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.95%. Comparing base (5c18f4b) to head (424c3f0).

Additional details and impacted files

@@                          Coverage Diff                           @@
##           fix-transport-retry-classification    #1013      +/-   ##
======================================================================
+ Coverage                               85.27%   94.95%   +9.67%     
======================================================================
  Files                                      58       58              
  Lines                                    5406     5406              
======================================================================
+ Hits                                     4610     5133     +523     
+ Misses                                    796      273     -523     

Flag	Coverage Δ
integration	91.86% <ø> (?)
unit	85.27% <ø> (ø)

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