Switch Cargo.toml's documentation link to docs.rs

[fitzgen]

No description provided.

[jonhoo]

Now that rust-lang/crates.io#676 has landed, you can actually just leave out documentation entirely, and it will default to docs.rs.

[coveralls]

Coverage increased (+3.2%) to 92.925% when pulling adb0296 on fitzgen:docs-in-cargo-toml into d591dc4 on gimli-rs:master.

[fitzgen]

Going to merge this PR.

Don't care if we ultimately remove the documentation line and rely on crates.io's default behavior or not, but it seems we shouldn't point to github pages anymore either way, so this PR is an improvement over the status quo.