Should `bazel_skylib_gazelle_plugin_setup()` live elsewhere?

[The instructions](http://bazel_skylib_gazelle_plugin_workspace/) mention to call the following:
```python
load("@bazel_skylib_gazelle_plugin//:setup.bzl", "bazel_skylib_gazelle_plugin_setup")

bazel_skylib_gazelle_plugin_setup()
```

This caused an error for me as I already had those [same initialization calls](https://github.com/bazelbuild/bazel-skylib/blob/60abca8fa2f970d4899857a46cb0586338dbf7a1/gazelle/setup.bzl#L27-L32) in my WORKSPACE. I wonder if we should amend the instructions to only call that if you do not already have a Gazelle extension installed. Alternatively, I wonder if the boilerplate that `bazel_skylib_gazelle_plugin_setup()` includes should actually live in the `bazel-gazelle` repository.

Start with the linked gazelle setup instructions and inspect the existing initialization calls in gazelle/setup.bzl at lines 27-32. Compare the documented bazel_skylib_gazelle_plugin_setup() flow with a WORKSPACE that already has a Gazelle extension, then determine whether the documentation should change or the boilerplate should move to bazel-gazelle; done means one approach is selected and reflected consistently.

Written by the indexing model from the issue text.