feat: add docs to plot/vega/padding/ctor

lib/node_modules/@stdlib/plot/vega/padding/ctor/README.md

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+<!--
+
+@license Apache-2.0
+
+Copyright (c) 2026 The Stdlib Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+-->
+
+# Padding
+
+> Padding constructor.
+
+<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->
+
+<section class="intro">
+
+</section>
+
+<!-- /.intro -->
+
+<!-- Package usage documentation. -->
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var Padding = require( '@stdlib/plot/vega/padding/ctor' );
+```
+
+#### Padding( \[options] )
+
+Returns a padding instance.
+
+```javascript
+var padding = new Padding();
+// returns <Padding>
+```
+
+The constructor accepts the following `options`:
+
+-   **bottom**: bottom padding (in pixels). Default: `0`.
+-   **left**: left padding (in pixels). Default: `0`.
+-   **right**: right padding (in pixels). Default: `0`.
+-   **top**: top padding (in pixels). Default: `0`.
+
+```javascript
+var padding = new Padding({
+    'bottom': 10,
+    'left': 20,
+    'right': 30,
+    'top': 40
+});
+// returns <Padding>
+
+var v = padding.top;
+// returns 40
+```
+
+* * *
+
+### Writable Properties
+
+<a name="prop-bottom"></a>
+
+#### Padding.prototype.bottom
+
+Bottom padding (in pixels). Default: `0`.
+
+```javascript
+var padding = new Padding();
+
+// Set:
+padding.bottom = 10;
+
+// Get:
+var v = padding.bottom;
+// returns 10
+```
+
+<a name="prop-left"></a>
+
+#### Padding.prototype.left
+
+Left padding (in pixels). Default: `0`.
+
+```javascript
+var padding = new Padding();
+
+// Set:
+padding.left = 10;
+
+// Get:
+var v = padding.left;
+// returns 10
+```
+
+<a name="prop-right"></a>
+
+#### Padding.prototype.right
+
+Right padding (in pixels). Default: `0`.
+
+```javascript
+var padding = new Padding();
+
+// Set:
+padding.right = 10;
+
+// Get:
+var v = padding.right;
+// returns 10
+```
+
+<a name="prop-top"></a>
+
+#### Padding.prototype.top
+
+Top padding (in pixels). Default: `0`.
+
+```javascript
+var padding = new Padding();
+
+// Set:
+padding.top = 10;
+
+// Get:
+var v = padding.top;
+// returns 10
+```
+
+* * *
+
+### Read-only Properties
+
+<a name="prop-properties"></a>
+
+#### Padding.prototype.properties
+
+List of enumerable properties.
+
+```javascript
+var padding = new Padding();
+
+var v = padding.properties;
+// returns [ 'bottom', 'left', 'right', 'top' ]
+```
+
+* * *
+
+### Methods
+
+<a name="method-to-json"></a>
+
+#### Padding.prototype.toJSON()
+
+Serializes a padding instance to a JSON object.
+
+```javascript
+var padding = new Padding({
+    'top': 10
+});
+
+var v = padding.toJSON();
+// returns { 'bottom': 0, 'left': 0, 'right': 0, 'top': 10 }
+```
+
+This method is implicitly invoked by `JSON.stringify`.
+
+```javascript
+var padding = new Padding({
+    'top': 10
+});
+
+var str = JSON.stringify( padding );
+// returns '{"bottom":0,"left":0,"right":0,"top":10}'
+```
+
+* * *
+
+### Events
+
+#### 'change'
+
+Emitted whenever a property value changes.
+
+```javascript
+var padding = new Padding();
+
+padding.on( 'change', onChange );
+
+padding.top = 10;
+
+function onChange( event ) {
+    console.log( 'Updated property: %s', event.property );
+}
+```
+
+</section>
+
+<!-- /.usage -->
+
+<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+
+<section class="notes">
+
+</section>
+
+<!-- /.notes -->
+
+<!-- Package usage examples. -->
+
+<section class="examples">
+
+* * *
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var Padding = require( '@stdlib/plot/vega/padding/ctor' );
+
+var padding = new Padding();
+console.log( padding.toJSON() );
+```
+
+</section>
+
+<!-- /.examples -->
+
+<!-- Section to include cited references. If references are included, add a horizontal rule *before* the section. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+
+<section class="references">
+
+</section>
+
+<!-- /.references -->
+
+<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->
+
+<section class="related">
+
+</section>
+
+<!-- /.related -->
+
+<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+
+<section class="links">
+
+</section>
+
+<!-- /.links -->