Add support to include links to the source code if available

Using the Nim documentation as an example, check out [this](https://nim-lang.org/docs/threads.html) link.

You can see that every method, variable or constant has a link directly to the source code in github.com. This makes it very easy to look at the actual implementation.

Other random languages I just checked. These also have such links handy.

[Crystal](https://crystal-lang.org/api/0.24.1/Tuple.html)
[Elixir](https://hexdocs.pm/elixir/Atom.html)
[Rust](https://doc.rust-lang.org/std/primitive.char.html)

Having access to the implementation by link in devdocs will make it super useful. Without it, I end up using the language docs even though devdocs is much cooler.

As an aside, I see you have a link to the original doc page that was scraped at the bottom so in theory I could click on that, find the function again and then click there. But most docs also have a permalink to each entry. It would be beneficial to include this permalink within devdocs as well so you could jump to the official documentation from right there.

Ideally, we have both these links handy within devdocs so there's no reason to go to the official docs.