docs: Document the transport-hook contract and the built-in HTTPX client

[vdusek]

Rewrites the HTTP clients concepts page around the three-layer hierarchy and the transport hooks, documents the built-in HTTPX client from #1004 (installation, sync/async examples, timeout semantics), and replaces the old HTTPX guide with "Build a custom HTTP client", which implements full AIOHTTP and requests clients including response adapters for the HttpResponse protocol.

Stack: #1022 (pipeline) → #1004 (HTTPX client) → #1013 (this). Split out of #1006.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.01%. Comparing base (4b5a50e) to head (ccfd59f).

Additional details and impacted files

@@ Coverage Diff @@## add-httpx-client #1013 +/- ##
=================================================
Coverage 95.01% 95.01% =================================================
Files 59 59 Lines 5501 5501 =================================================
Hits 5227 5227 Misses 274 274 

Flag	Coverage Δ
integration	92.05% <ø> (-0.02%)	⬇️
unit	85.96% <ø> (ø)

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