Fix rustdoc links, especially around petgraph

[kesyog]

Link to rustdoc items by name rather than using hard-coded direct links, as recommended by https://doc.rust-lang.org/rustdoc/write-documentation/linking-to-items-by-name.html. This should hopefully fix an issue where the generated documentation links to an older version of petgraph.

[kesyog]

Heads up @mitchmindtree