Add missing core_thread.html to the list - previously nothing depende…

[maxhaton]

…d on it so the file was never emitted

If this works then I will update the documentation to be more like std.algorithm i.e. list the submodules

[dlang-bot]

Thanks for your pull request, @maxhaton!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21717	critical	[Oh No! Page Not Found]

⚠️⚠️⚠️ Warnings ⚠️⚠️⚠️

• Regression or critical bug fixes should always target the stable branch. Learn more about rebasing to stable or the D release process.

To target stable perform these two steps:

1. Rebase your branch to upstream/stable:

git rebase --onto upstream/stable upstream/master

2. Change the base branch of your PR to stable

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#3652"

[Moth-Tolias]

https://dlang.org/phobos/core_thread.html remains page not found.