DMD performance dashboard is live now

[Abul Hossain Khan]

Hi everyone,

Quick update on the performance regression publisher project from 
GSoC. The dashboard is now up and running at 
https://perf.dlang.org

Every merge to dmd master gets measured by the perf workflow, the 
result goes into a data repo 
https://github.com/dlang/dmd-perf-data and the web page is 
showing those data.

Two improvements also went into the PR bot after GSoC:

- https://github.com/dlang/dmd/pull/23834 - The PR check was 
reusing base's PGO profile for the head build, so some PRs got 
big deltas that weren't real (one showed +7.49% on hello.d, after 
merge the master run said 0.00%). Now both sides train their own 
profile, so the numbers are a lot more accurate.
- https://github.com/dlang/dmd/pull/23884 - base and head are now 
built in parallel, nearly halves build time.

I will keep an eye on the dashboard and look into any spikes, 
that will be be interesting.
Thanks to Dennis for mentoring the whole thing, and Mike for 
setting things up, and to the whole D community. And as always, 
feel free to leave any feedback or suggestions!