Update links in cargo-contract docs

[Daanvdplas]

The links provided in the cargo-contract docs point towards old line numbers. E.g. a link that is meant to point to a dispatchable now points to something random (due to the code that has been added).

[SkymanOne]

Can you please send links to pages that contain errors?

[Daanvdplas]

All the links to dispatchables here: https://github.com/paritytech/cargo-contract/blob/master/docs/extrinsics.md

[HCastano]

In the future we should be using the "permalink" generated by GitHub instead.

Code may become outdated, but at least it'll always point to the intended location