Fix 22851 - Add source reference to std.sumtype

[ghost]

Add Source: $(PHOBOSSRC std/sumtype.d) to std.sumtype's module documentation.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22851	trivial	Missing reference to std.sumtype's source in the latter's documentation

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 + phobos#8403"