docs: add custom HTTP clients concept page and HTTPX guide #294

Job	Run time
PR title check	4s
Docstrings checks / Docstrings checks	20s
Code checks / Actions lint check	31s
Code checks / Spell check	8s
Doc checks / Doc checks / Docs check	1m 21s
Code checks / Lint check / Lint check (3.11)	10s
Code checks / Type check / Type check (3.13)	15s
Code checks / Type check / Type check (3.14)	10s
Code checks / Type check / Type check (3.11)	11s
Code checks / Lint check / Lint check (3.14)	15s
Code checks / Lint check / Lint check (3.13)	13s
Tests / Unit tests / Unit tests (windows-latest, 3.14)	1m 1s
Code checks / Lint check / Lint check (3.12)	10s
Code checks / Type check / Type check (3.12)	11s
Tests / Unit tests / Unit tests (windows-latest, 3.12)	1m 5s
Tests / Unit tests / Unit tests (windows-latest, 3.11)	57s
Tests / Unit tests / Unit tests (ubuntu-latest, 3.11)	36s
Tests / Unit tests / Unit tests (windows-latest, 3.13)	57s
Tests / Unit tests / Unit tests (ubuntu-latest, 3.12)	41s
Tests / Unit tests / Unit tests (ubuntu-latest, 3.13)	39s
Tests / Unit tests / Unit tests (ubuntu-latest, 3.14)	43s
Tests / Integration tests / Integration tests (ubuntu-latest, 3.11)	1m 1s
Tests / Integration tests / Integration tests (ubuntu-latest, 3.14)	58s
	12m 37s

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