Undocumented custom domain setting

As discussed in #724, we support a custom domain setting but there is no mention within the `Configuration.rst` documentation.

we support:

```
[tool.semantic_release.remote]
type = "github"

# if you want it to dynamically read from the env
domain = { env: "GITHUB_SERVER_URL" }

# or hardcoded
domain = "dev.asure"
```