[TASK] Add missing page.meta attribute sub-property (#1337)

The more generic `attribute` sub-property can be used to set the attribute
to any arbitrary value. An example is already available for the
Open Graph meta tags. Therefore, it is more generic than the
`httpEquivalent` sub-property.

Releases: main, 12.4
(cherry picked from commit 8c43fe7)

Author: brotkrueml

Documentation/TopLevelObjects/Page/Index.rst

@@ -596,9 +596,14 @@ Properties
 
         For each key the following sub-properties are available:
 
+        `attribute`
+            Sets the attribute for the meta tag. If it is not defined, the
+            default :html:`name` is used.
+
         `httpEquivalent`
             If set to 1, the :html:`http-equiv` attribute is used in the meta
             tag instead of the :html:`name` attribute. Default: 0.
+
         `replace`
             If set to 1, the tag will replace the one set earlier by a plugin. If set
             to 0 (default), the meta tag generated by the plugin will be used. If