Fix issue 21781 - Add core/sync/package.d for documentation

[dkorpel]

The ddoc text is bare, I'm not familiar with the module so I'm not writing a comprehensive description yet, but this should fix the 404 and allow you to import core.sync;.

Makefiles updated similar to #2941

[dlang-bot]

Thanks for your pull request and interest in making D better, @dkorpel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21781	minor	[Oh No! Page Not Found] Links to core libs from Better C

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + druntime#3836"

[ljmf00]

Please rebase.

[dkorpel]

I'll just merge myself since the vide-d buildkite error is an issue with vibe-d itself.