chore(css): Add mdn_url for newly documented features (#937)

Author: skyclouds2001

css/properties.json

@@ -6436,7 +6436,8 @@
     "appliesto": "blockContainersExceptMultiColumnContainers",
     "computed": "asSpecified",
     "order": "perGrammar",
-    "status": "standard"
+    "status": "standard",
+    "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/line-clamp"
   },
   "line-height": {
     "syntax": "normal | <number> | <length> | <percentage>",

css/selectors.json

@@ -1085,7 +1085,8 @@
       "Pseudo-elements",
       "Selectors"
     ],
-    "status": "experimental"
+    "status": "experimental",
+    "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/::details-content"
   },
   "::file-selector-button": {
     "syntax": "::file-selector-button",