fix: docs/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-SHARP-5922108

Author: snyk-bot

docs/package.json

@@ -6,7 +6,7 @@
   "dependencies": {
     "gatsby": "^3.0.0",
     "gatsby-plugin-google-analytics": "^2.1.16",
-    "gatsby-plugin-manifest": "^3.13.0",
+    "gatsby-plugin-manifest": "^5.10.0",
     "gatsby-plugin-offline": "^2.0.25",
     "gatsby-plugin-react-helmet": "^3.0.10",
     "gatsby-plugin-sass": "^2.0.11",