Update promotion.rst to explain mode promote #29851

Job	Run time
Documentation	54s
Nix (macos-latest)	2m 5s
Nix (ubuntu-latest)	52s
Format	1m 2s
Build (macos-latest, 5.1.x)	14m 56s
Build microbenchmarks	2m 31s
Build (ubuntu-latest, 5.1.x)	8m 38s
Build (ocaml-variants.5.4.0+trunk, ubuntu-latest, true)	1m 29s
Coq 8.16.1	7m 14s
Create local opam switch (macos-latest, 5)	5m 14s
Build monorepo benchmark docker image	1m 36s
Create local opam switch (macos-latest, 4.14)	5m 47s
Build (4.14.x, ubuntu-latest, true)	1m 27s
Wasm_of_ocaml	10m 30s
Create local opam switch (ubuntu-latest, 5)	4m 49s
Create local opam switch (ubuntu-latest, 4.14)	4m 47s
Build (4.14.x, macos-latest, true)	2m 39s
Build (5.1.x, macos-13, true)	5m 6s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	14m 55s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	8m 23s
Build (4.08.x, ubuntu-latest, true)	1m 32s
Build (4.04.x, ubuntu-latest, true, true)	4m 26s
Build (4.02.x, ubuntu-latest, true)	3m 54s
	1h 54m 46s

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