Document tracking allocator

[jdm]

• build with --features servo-allocator/tracking-allocator
• set UNTRACKED_LOG_FILE env var to a file name
• run a website then open about:memory and report memory usage
• run script from https://gist.github.com/jdm/f1eb95a2bc8b86d886c78e47e16d4fd4 with python3 untracked-allocations.py $UNTRACKED_LOG_FILE

[jdm]

Script now lives at https://github.com/servo/servo/blob/main/etc/untracked-allocations.py

[LiamillionSS]

It looks like the feature is now servo-allocator/allocation-tracking, when attempting to run the build:

package `servoshell` depends on `servo-allocator` with feature `tracking-allocator` but `servo-allocator` does not have that feature.
 available features: allocation-tracking, backtrace, libc, log, use-system-allocator