Set up / document memory usage benchmarks #313

From cursory evaluation glide appears to use more memory than firefox; while we generally will have slightly higher memory usage as we do more things than stock Firefox, we should be confident that we are not introducing any significant memory usage regressions.

Firefox likely have their own dev tooling for measuring and diagnosing memory usage, so we should find out how that works and document it in the contributing docs. Then we can work on reducing our overall memory footprint.