build

Add jemalloc section admonition #134

Job	Run time
build-std (build_std)	39s
no_std (ubuntu-latest, stable)	7s
no_main-win (stable)	1m 16s
build-std (no_main/nix)	38s
Default (ubuntu-latest, stable)	6s
no_std (ubuntu-latest, nightly)	11s
no_main-win (nightly)	30s
Default (ubuntu-latest, nightly)	16s
no_std (macos-latest, stable)	11s
Default (macos-latest, stable)	11s
no_std (macos-latest, nightly)	16s
Default (macos-latest, nightly)	18s
no_std-win (stable)	1m 6s
Default (windows-latest, stable)	1m 1s
no_std-win (nightly)	28s
Default (windows-latest, nightly)	23s
	7m 37s

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