Fixed broken documentation links for Directed and Undirected enums.

[john-hartley]

I'm not sure if there's an automated way to test for broken documentation links, as I'm new to Rust, so I've verified locally using cargo doc --open.

Thanks for the crate all the same. I was tearing my hair out trying to figure out how to work with graphs.

[ABorgna]

Thanks for checking!
There is cargo-deadlinks, which shows there are still other broken links. Do you want to fix them?

We could think about adding that check to the CI at some point.

[john-hartley]

No problem!

I'm happy to fix the other errors, but I've run into a problem installing cargo-deadlinks, for which I've created an issue in one of its dependencies. Once I get that sorted, then I'll fix the rest.

[john-hartley]

I've fixed the remaining problems reported by cargo deadlinks.

Where I've added #[doc(hidden)] to some methods it's because the traits that declare those methods have also marked them as #[doc(hidden)]. deadlinks was reporting that they don't exist because #[doc(hidden)] was applied to the trait but not the implementation.

[ABorgna]

Great ☺
Thanks for the contribution.