Add css layer doc

doc/theming/styled/csslayer/SpecificityDoc.vue

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+<template>
+    <DocSectionText v-bind="$attrs">
+        <p>
+            The <i>@layer</i> is a standard CSS feature to define cascade layers for a customizable order of precedence. If you need to become more familiar with layers, visit the documentation at
+            <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/@layer" class="doc-link">MDN</a> to begin with. In styled mode, when the <i>cssLayer</i> option is enabled at theme configuration, PrimeVue wraps the built-in style classes under
+            the <i>primevue</i> cascade layer to make the library styles easy to override. CSS in your app without a layer has the highest CSS specificity, so you'll be able to override styles regardless of the location or how strong a class is
+            written. The <i>cssLayer</i> is disabled by default to avoid compatibility issues with 3rd party CSS libraries which requires a layer configuration for compatibility that is discussed in the next reset section.
+        </p>
+        <p>
+            For example, let's assume you need to remove the rounded borders of the InputSwitch component defined by the theme in use. In order to achieve this, <i>.p-inputswitch .p-inputswitch-slider</i> selector needs to be overriden. Without the
+            layers, we'd have to write a stronger css or use <i>!important</i> however, with layers, this does not present an issue as your CSS can always override PrimeVue with a more straightforward class name such as <i>my-switch-slider</i>.
+            Another advantage of this approach is that it does not force you to figure out the built-in class names of the components.
+        </p>
+        <div class="card flex justify-content-center">
+            <InputSwitch v-model="checked" :pt="{ slider: 'my-switch-slider' }" />
+        </div>
+        <DocSectionCode :code="code" hideToggleCode hideStackBlitz />
+        <p>Layers also make it easier to use CSS Modules, view the CSS Modules guide for examples.</p>
+    </DocSectionText>
+</template>
+
+<script>
+export default {
+    data() {
+        return {
+            checked: false,
+            code: {
+                basic: `
+<template>
+    <InputSwitch v-model="checked" :pt="{ slider: 'my-switch-slider' }" />
+</template>
+
+<script>
+import { ref } from "vue";
+
+const checked = ref(false);
+<\/script>
+
+<style>
+.my-switch-slider {
+    border-radius: 0;
+}
+
+.my-switch-slider:before {
+    border-radius: 0;
+}
+</style>
+`
+            }
+        };
+    }
+};
+</script>
+
+<style>
+.my-switch-slider {
+    border-radius: 0;
+}
+
+.my-switch-slider:before {
+    border-radius: 0;
+}
+</style>