Move jemalloc to a legacy techniques section #139

Job	Run time
no_main-win (stable)	57s
no_std (ubuntu-latest, stable)	11s
build-std (build_std)	44s
no_main-win (nightly)	36s
Default (ubuntu-latest, stable)	15s
build-std (no_main/nix)	41s
no_std (ubuntu-latest, nightly)	14s
Default (ubuntu-latest, nightly)	14s
no_std (macos-latest, stable)	16s
no_std-win (stable)	50s
Default (macos-latest, stable)	17s
no_std (macos-latest, nightly)	16s
no_std-win (nightly)	43s
Default (macos-latest, nightly)	17s
Default (windows-latest, stable)	49s
Default (windows-latest, nightly)	35s
	7m 55s

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