Update promotion.rst to explain mode promote #29803

Job	Run time
Documentation	55s
Build (macos-latest, 5.1.x)	13m 43s
Coq 8.16.1	7m 16s
Build (ubuntu-latest, 5.1.x)	8m 30s
Build microbenchmarks	2m 31s
Build (ocaml-variants.5.4.0+trunk, ubuntu-latest, true)	1m 28s
Create local opam switch (macos-latest, 5)	7m 38s
Build (4.14.x, ubuntu-latest, true)	1m 29s
Create local opam switch (macos-latest, 4.14)	5m 53s
Format	1m 3s
Build (4.14.x, macos-latest, true)	3m 1s
Build monorepo benchmark docker image	1m 44s
Create local opam switch (ubuntu-latest, 5)	5m 7s
Build (5.1.x, macos-13, true)	3m 54s
Nix (macos-latest)	2m 13s
Create local opam switch (ubuntu-latest, 4.14)	4m 58s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	15m 7s
Nix (ubuntu-latest)	53s
Wasm_of_ocaml	10m 26s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	8m 30s
Build (4.08.x, ubuntu-latest, true)	1m 24s
Build (4.04.x, ubuntu-latest, true, true)	4m 32s
Build (4.02.x, ubuntu-latest, true)	3m 53s
	1h 56m 8s

Provide feedback