docs(CONTRIBUTING): Add section on how to interact with local changes

[MagnusHJensen]

This PR adds a new section to the CONTRIBUTING.md docs, that explains what one needs to do to verify the local changes made, the most prominent step is the replace directive, which should help out new contributors.

Some formatting changes, that my editor picked up on, which seems to just be correcting some earlier mistakes, the markdown looks the same.

[DeathHound6]

Another approach I've used instead of modifying go.mod, is go workspaces

$ go work init [./path/to/local/discordgo]
$ go work use [./path/to/another/local/module]

When a go workspace is created, the path to the go.work file, by default, is assigned into the GOWORK env var. This is how go knows which modules to use instead of the published remote variants
If you instead wish to use the module from go.pkg.dev, then set GOWORK=off