Link to short commit SHA on front-page

[maximecb]

I think it would be useful to have link with the short-form commit SHA at the top of the speed.yjit.org front page next to the date, e.g.

YJIT metrics from the yjit-bench as of 2024-10-07 (8d1906e)

Otherwise maybe we could make the date directly link to the Ruby commit. I realize that this information is already present in the Ruby version string, but it's maybe not as immediately intuitive as next to the date when the benchmarks were run, so maybe we can have both?

WDYT? Feel free to just close this issue if you disagree.