docs: add custom HTTP clients concept page and HTTPX guide (#658) #76

Job	Run time
Code checks / Spell check	8s
Code checks / Actions lint check	30s
Doc checks / Doc checks / Docs check	1m 12s
Docstrings checks / Docstrings checks	23s
Code checks / Type check / Type check (3.12)	14s
Code checks / Type check / Type check (3.14)	11s
Code checks / Type check / Type check (3.11)	10s
Code checks / Type check / Type check (3.13)	16s
Code checks / Lint check / Lint check (3.11)	10s
Code checks / Lint check / Lint check (3.12)	10s
Code checks / Lint check / Lint check (3.14)	14s
Code checks / Lint check / Lint check (3.13)	10s
Tests	0s
Release prepare	-1s
Changelog update	-1s
PyPI publish	-1s
Doc release post publish	0s
Doc release / Doc release	1m 26s
	5m 11s

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