Docs: Add sitemap and robots.txt

Author: sequba

docs/.vuepress/config.js

@@ -63,6 +63,11 @@ module.exports = {
   ],
   base: '/',
   plugins: [
+    ['sitemap', {
+      hostname: 'https://hyperformula.handsontable.com',
+      exclude: ['/404.html'],
+      changefreq: 'weekly'
+    }],
     searchBoxPlugin,
     ['container', examples()],
     {

docs/.vuepress/public/robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+Allow: /
+
+Sitemap: https://hyperformula.handsontable.com/sitemap.xml