Merge pull request #56961 from danielqsj/remove_fed_doc

Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Remove unused federation docs

**What this PR does / why we need it**:

Federation docs are already in project [federation](https://github.com/kubernetes/federation).
And /hack/generate-docs.sh here do not generate federation docs right now.
So it's time to remove these docs.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #56958 

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```

docs/admin/federation-controller-manager.md

@@ -1,3 +0,0 @@
-This file is autogenerated, but we've stopped checking such files into the
-repository to reduce the need for rebases. Please run hack/generate-docs.sh to
-populate this file.

docs/admin/kubefed.md

@@ -1,3 +0,0 @@
-This file is autogenerated, but we've stopped checking such files into the
-repository to reduce the need for rebases. Please run hack/generate-docs.sh to
-populate this file.

docs/admin/kubefed_init.md

@@ -1,3 +0,0 @@
-This file is autogenerated, but we've stopped checking such files into the
-repository to reduce the need for rebases. Please run hack/generate-docs.sh to
-populate this file.

docs/admin/kubefed_join.md

@@ -1,3 +0,0 @@
-This file is autogenerated, but we've stopped checking such files into the
-repository to reduce the need for rebases. Please run hack/generate-docs.sh to
-populate this file.

docs/admin/kubefed_options.md

@@ -1,3 +0,0 @@
-This file is autogenerated, but we've stopped checking such files into the
-repository to reduce the need for rebases. Please run hack/generate-docs.sh to
-populate this file.

docs/admin/kubefed_unjoin.md

@@ -1,3 +0,0 @@
-This file is autogenerated, but we've stopped checking such files into the
-repository to reduce the need for rebases. Please run hack/generate-docs.sh to
-populate this file.

docs/admin/kubefed_version.md

@@ -1,3 +0,0 @@
-This file is autogenerated, but we've stopped checking such files into the
-repository to reduce the need for rebases. Please run hack/generate-docs.sh to
-populate this file.