Bump @primer/css from 14.4.0 to 15.0.0

Bumps [@primer/css](https://github.com/primer/css) from 14.4.0 to 15.0.0.
- [Release notes](https://github.com/primer/css/releases)
- [Changelog](https://github.com/primer/css/blob/master/CHANGELOG.md)
- [Commits](https://github.com/primer/css/compare/v14.4.0...v15.0.0

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package-lock.json

 {
   "name": "just-the-docs",
-  "version": "0.3.0",
+  "version": "0.3.1",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {
@@ -276,9 +276,9 @@
       }
     },
     "@primer/css": {
-      "version": "14.4.0",
-      "resolved": "https://registry.npmjs.org/@primer/css/-/css-14.4.0.tgz",
-      "integrity": "sha512-o9DwcAH43jZNDC/rPLjsYTU/I1LsHIgIQlSO3X8vZV2u65LmWeA5jHI2cSadZYN3N0Gm0Soh0zVYuhh7kMskXA==",
+      "version": "15.0.0",
+      "resolved": "https://registry.npmjs.org/@primer/css/-/css-15.0.0.tgz",
+      "integrity": "sha512-9jMRl9rOsdy8kE7AW0tM3tyom1PTV7JMT1TCE9FgZ8HDM2xAaM5+apb0Ss4RZ7+5eAZbW9ayGX+iXyL6aHPhgg==",
       "dev": true,
       "requires": {
         "@primer/octicons": "^9.1.1"

package.json

@@ -6,7 +6,7 @@
   "license": "MIT",
   "bugs": "https://github.com/pmarsceill/just-the-docs/issues",
   "devDependencies": {
-    "@primer/css": "^14.4.0",
+    "@primer/css": "^15.0.0",
     "prettier": "^2.0.5",
     "stylelint": "^13.6.1",
     "stylelint-config-prettier": "^8.0.2",