Add instructions for configuration of Emacs #21310

Job	Run time
cargo fmt	23s
cargo clippy	2m 17s
cargo test (windows)	4m 13s
cargo test (wasm)	2m 1s
cargo fuzz	4m 20s
cargo test (linux)	7m 32s
Determine changes	11s
test scripts	1m 11s
benchmarks	14m 20s
cargo udeps	3m 13s
mkdocs	1m 23s
pre-commit	52s
python package	1m 13s
formatter instabilities and black similarity	0s
test ruff-lsp	20s
ecosystem	38s
	44m 7s

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