Add workflow to build and profile third-party repo

[ashmaroli]

Summary

A GitHub Actions Workflow to build a third-party repository and profile memory usage of the build as well.
The third-party repository is a slightly modified fork of public repo https://github.com/tomjoht/tomjoht.github.io which was earlier used in the (now retired) Utterson check-runs.

• Using the fork means the contents don't change over time even if the original upstream repository adds new content.
• The memory-profiling script only traces files with the stub lib/jekyll/ in order to better expose relevant information.

[DirtyF]

Is it intended to replace the profile docs on Travis?

[ashmaroli]

Is it intended to replace the profile docs on Travis?

Not really.
The profile-docs and memprof jobs on Travis and AppVeyor uses jekyllrb's source as subject. This workflow uses a third-party repository which contains a huge lot more pages and documents than jekyllrb's source directory, which will give us additional insight on the various optimizations / changes.

[DirtyF]

💡 A bot that sends a benchmark summary on every PR that modifies the jekyll lib 🔥

[ashmaroli]

A bot that sends a benchmark summary on every PR that modifies the jekyll lib

That's a great idea @DirtyF!!
I'll look into having a workflow for that..

Merging this for the time being to be used in open PRs about optimizations..
@jekyllbot: merge +dev