Adds a link to the source code of the current module

[denizzzka]

Adds a link to the source, without editing enabled. I am found that it is very convenient to have link here.

(Yes, I know there's usually a link to the source at the bottom of the page)

[dlang-bot]

Thanks for your pull request and interest in making D better, @denizzzka! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

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