Update nextra monorepo to v3.0.0-alpha.13 (#76)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package.json

@@ -30,8 +30,8 @@
     "autoprefixer": "^10.4.16",
     "husky": "^8.0.3",
     "next": "^13.5.6",
-    "nextra": "3.0.0-alpha.12",
-    "nextra-theme-docs": "3.0.0-alpha.12",
+    "nextra": "3.0.0-alpha.13",
+    "nextra-theme-docs": "3.0.0-alpha.13",
     "postcss": "^8.4.21",
     "prettier": "^2.8.8",
     "pretty-quick": "^3.3.1",