docs: Add troubleshooting guide for 421 Invalid Host Header error #1540

Job	Run time
checks / readme-snippets	13s
checks / pre-commit	22s
checks / test (3.12, lowest-direct, ubuntu-latest)	10m 16s
checks / test (3.10, lowest-direct, windows-latest)	10m 16s
checks / test (3.10, lowest-direct, ubuntu-latest)	10m 16s
checks / test (3.11, locked, ubuntu-latest)	10m 15s
checks / test (3.13, lowest-direct, windows-latest)	10m 16s
checks / test (3.11, lowest-direct, ubuntu-latest)	10m 17s
checks / test (3.12, lowest-direct, windows-latest)	10m 15s
checks / test (3.10, locked, ubuntu-latest)	10m 17s
checks / test (3.14, locked, ubuntu-latest)	10m 16s
checks / test (3.12, locked, windows-latest)	10m 16s
checks / test (3.12, locked, ubuntu-latest)	10m 17s
checks / test (3.14, lowest-direct, ubuntu-latest)	10m 17s
checks / test (3.14, locked, windows-latest)	10m 16s
checks / test (3.13, lowest-direct, ubuntu-latest)	10m 15s
checks / test (3.11, lowest-direct, windows-latest)	10m 15s
checks / test (3.14, lowest-direct, windows-latest)	10m 16s
checks / test (3.13, locked, windows-latest)	10m 16s
checks / test (3.10, locked, windows-latest)	10m 15s
checks / test (3.13, locked, ubuntu-latest)	10m 15s
checks / test (3.11, locked, windows-latest)	10m 17s
all-green	5s
	3h 25m 59s

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