HTML Report: simplify baseline cycles display in report and show confidence interval pct

From https://github.com/bytecodealliance/sightglass/pull/295#pullrequestreview-3589220910

> Also, it would be nice to show the confidence interval as a percentage (in addition or instead of the absolute value) since, at least for me personally, the absolute values are much less useful at a glance than the percentage. Maybe the "Performance" column should include a +/- 1.23% or something? Lots of ways we could display this, don't care too much about the details, more just that we try to make things as minimally noisy and easy to read at a glance as possible.