docs: document config.kubernetes.io/local-config annotation (#6133)

* docs: document config.kubernetes.io/local-config annotation

Adds a Concepts page covering the three documented use cases:
Replacement data source, name-reference anchor across objects, and
KRM Function shared configuration. Cross-references the existing
manifest-annotations.md and the Kubernetes well-known annotations
page.

Signed-off-by: jawwad-ali <33836051+jawwad-ali@users.noreply.github.com>

* docs: drop implementation details and deprecated kustomize cfg note

Per koba1t review feedback on the local-config Concepts page:
implementation details (IsLocalConfig filter, DropLocalNodes helper,
source-file references) belong in the code rather than user docs, and
`kustomize cfg` is deprecated and should not be promoted to new users.
Removes the "How kustomize processes local-config resources" section.

Signed-off-by: jawwad-ali <33836051+jawwad-ali@users.noreply.github.com>

---------

Signed-off-by: jawwad-ali <33836051+jawwad-ali@users.noreply.github.com>

site/content/en/docs/Concepts/local-config.md

@@ -0,0 +1,118 @@
+---
+title: "Local Configuration"
+linkTitle: "Local Configuration"
+weight: 70
+description: >
+  What is the config.kubernetes.io/local-config annotation?
+---
+
+[well-known annotations]: https://kubernetes.io/docs/reference/labels-annotations-taints/#config-kubernetes-io-local-config
+[manifest-annotations.md]: https://github.com/kubernetes-sigs/kustomize/blob/master/cmd/config/docs/api-conventions/manifest-annotations.md
+
+The `config.kubernetes.io/local-config` annotation marks a resource as
+local to the build: kustomize reads it like any other input, but omits
+it from the rendered output. The annotation is part of the [well-known
+annotations] shared across the KRM tooling ecosystem.
+
+A resource is local-config when it carries the annotation with the
+value `"true"`:
+
+```
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: settings
+  annotations:
+    config.kubernetes.io/local-config: "true"
+data:
+  region: us-east-1
+```
+
+Setting the annotation to `"false"`, or omitting it entirely, keeps
+the resource in the output.
+
+## Why it is useful
+
+Local-config resources let you keep build-time configuration alongside
+the resources you intend to apply, without having to manage that
+configuration in a separate file tree. They participate in name
+references and can be consumed by transformers, generators, and
+functions, but they never reach the cluster.
+
+### As a data source for a Replacement
+
+A `Replacement` can read fields from a local-config resource and copy
+them into one or more target resources. The source object exists only
+to carry the values:
+
+```
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: image-source
+  annotations:
+    config.kubernetes.io/local-config: "true"
+data:
+  image: registry.example.com/app:v1.2.3
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+  - deployment.yaml
+replacements:
+  - source:
+      kind: ConfigMap
+      name: image-source
+      fieldPath: data.image
+    targets:
+      - select:
+          kind: Deployment
+          name: app
+        fieldPaths:
+          - spec.template.spec.containers.0.image
+```
+
+The rendered output contains the patched `Deployment` only; the
+`ConfigMap` is dropped.
+
+### As a name-reference anchor across multiple objects
+
+When several resources share the same name fragment (for example, a
+hostname that appears in an `Ingress`, a `Secret`, and a `Deployment`
+env var), a local-config resource can serve as the canonical name. A
+`namePrefix` applied to the build then propagates to every reference
+through kustomize's name-reference machinery, while the anchor itself
+is removed from the output.
+
+A complete worked example lives in
+[`api/krusty/namereference_test.go`][nameref-test] under
+`TestIssue4884_UseLocalConfigAsNameRefSource`.
+
+[nameref-test]: https://github.com/kubernetes-sigs/kustomize/blob/master/api/krusty/namereference_test.go
+
+### As shared configuration for KRM Functions
+
+A function declared in `transformers:` or `generators:` is itself a
+KRM resource. Marking it local-config keeps the function definition in
+the kustomization, where it can be patched by overlays, without
+emitting the function spec to the cluster:
+
+```
+apiVersion: example.com/v1
+kind: ValidatingTransformer
+metadata:
+  name: enforce-labels
+  annotations:
+    config.kubernetes.io/local-config: "true"
+    config.kubernetes.io/function: |
+      container:
+        image: registry.example.com/fn-enforce-labels:v1
+spec:
+  required:
+    - app.kubernetes.io/name
+```
+
+## See also
+
+- [Kubernetes well-known annotations][well-known annotations]
+- [Manifest annotations reference][manifest-annotations.md]