docs: add custom HTTP clients concept page and HTTPX guide #296

Job	Run time
PR title check	6s
Docstrings checks / Docstrings checks	21s
Code checks / Actions lint check	32s
Code checks / Spell check	4s
Doc checks / Doc checks / Docs check	1m 25s
Code checks / Lint check / Lint check (3.11)	10s
Code checks / Type check / Type check (3.13)	12s
Code checks / Type check / Type check (3.14)	13s
Code checks / Lint check / Lint check (3.13)	12s
Code checks / Lint check / Lint check (3.14)	11s
Code checks / Type check / Type check (3.12)	12s
Code checks / Lint check / Lint check (3.12)	13s
Tests / Unit tests / Unit tests (ubuntu-latest, 3.14)	43s
Code checks / Type check / Type check (3.11)	11s
Tests / Unit tests / Unit tests (windows-latest, 3.11)	57s
Tests / Unit tests / Unit tests (ubuntu-latest, 3.13)	44s
Tests / Unit tests / Unit tests (windows-latest, 3.14)	57s
Tests / Unit tests / Unit tests (ubuntu-latest, 3.12)	44s
Tests / Unit tests / Unit tests (windows-latest, 3.12)	58s
Tests / Unit tests / Unit tests (ubuntu-latest, 3.11)	42s
Tests / Unit tests / Unit tests (windows-latest, 3.13)	1m 0s
Tests / Integration tests / Integration tests (ubuntu-latest, 3.11)	1m 2s
Tests / Integration tests / Integration tests (ubuntu-latest, 3.14)	1m 22s
	13m 11s

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