Move jemalloc to a legacy techniques section #140

Job	Run time
no_std (ubuntu-latest, stable)	11s
no_std-win (stable)	1m 1s
no_std (ubuntu-latest, nightly)	13s
build-std (build_std)	42s
no_std-win (nightly)	35s
no_std (macos-latest, stable)	15s
Default (ubuntu-latest, stable)	13s
build-std (no_main/nix)	41s
no_std (macos-latest, nightly)	14s
Default (ubuntu-latest, nightly)	13s
no_main-win (stable)	30s
Default (macos-latest, stable)	24s
Default (macos-latest, nightly)	17s
no_main-win (nightly)	35s
Default (windows-latest, stable)	1m 17s
Default (windows-latest, nightly)	32s
	7m 53s

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