Add sitemap (techlore#124)

I have added a plugin to generate a sitemap for each page/ app entry in
the project. On every build it will generate the sitemap at
`/sitemap.xml`

Close techlore#121

Author: tomkonidas

gatsby-config.js

@@ -1,6 +1,7 @@
 module.exports = {
   siteMetadata: {
     title: `Plexus`,
+    siteUrl: `https://plexus.techlore.tech`,
     description: `Remove the fear of Android app compatibility on de-Googled devices.`,
     author: `@tomkonidas`,
   },
@@ -52,6 +53,7 @@ module.exports = {
         fieldName: "slug",
       },
     },
+    `gatsby-plugin-sitemap`,
     // this (optional) plugin enables Progressive Web App + Offline functionality
     // To learn more, visit: https://gatsby.dev/offline
     // `gatsby-plugin-offline`,

package-lock.json

@@ -13,6 +13,7 @@
     "gatsby-plugin-react-helmet": "^3.7.0",
     "gatsby-plugin-react-svg": "^3.0.0",
     "gatsby-plugin-sharp": "^2.11.2",
+    "gatsby-plugin-sitemap": "2.11.0",
     "gatsby-plugin-slug-field": "^0.2.0",
     "gatsby-source-filesystem": "^2.8.1",
     "gatsby-transformer-csv": "^2.7.0",