Add Adobe Lightroom to list of XMP Specialized Namespaces

Since the Lightroom namespace is widely used, it would be helpful to include in the list of specialized namespaces on https://developer.adobe.com/xmp/docs/XMPNamespaces/

- The namespace URI is http://ns.adobe.com/lightroom/1.0/
- The preferred namespace prefix is lr

| Name | Description | Type |
| ---------------------- | -------------------------------------- | --------------------- |
| lr:hierarchicalSubject | Adobe Lightroom hierarchical keywords. | Ordered array of text |
| lr:privateRTKInfo | Adobe Lightroom private RTK info. | text |
| lr:WeightedFlatSubject | Adobe Lightroom weighted keywords. | text |

Start by locating the source for the XMP Specialized Namespaces list linked in the issue and read its surrounding entries and contribution instructions. Add Adobe Lightroom with URI http://ns.adobe.com/lightroom/1.0/, prefix lr, and the three listed properties, then verify the generated documentation page includes the new namespace.

Written by the indexing model from the issue text.