docs(site): downgrade sirv, commonjs plugin, preserve rollup signatures, minor seo tweak (#950)

* fix: sirv, commonjs plugin, add rollup sig

* replace deprecated rollup plugins

* remove dollarsign

site/rollup.config.js

@@ -1,8 +1,8 @@
+import babel from '@rollup/plugin-babel';
 import commonjs from '@rollup/plugin-commonjs';
 import json from '@rollup/plugin-json';
 import resolve from '@rollup/plugin-node-resolve';
-import babel from 'rollup-plugin-babel';
-import replace from 'rollup-plugin-replace';
+import replace from '@rollup/plugin-replace';
 import svelte from 'rollup-plugin-svelte';
 import { terser } from 'rollup-plugin-terser';
 import config from 'sapper/config/rollup.js';
@@ -42,12 +42,15 @@ export default {
                 hydratable: true,
                 emitCss: true
             }),
-            resolve(),
+            resolve({
+                browser: true,
+                dedupe: ['svelte']
+            }),
             commonjs(),
 
             legacy && babel({
                 extensions: ['.js', '.mjs', '.html', '.svelte'],
-                runtimeHelpers: true,
+                babelHelpers: 'runtime',
                 exclude: ['node_modules/@babel/**'],
                 presets: [
                     ['@babel/preset-env', {
@@ -68,6 +71,7 @@ export default {
 
             json()
         ],
+        preserveEntrySignatures: false,
         onwarn,
     },
 
@@ -83,13 +87,16 @@ export default {
                 generate: 'ssr',
                 dev
             }),
-            resolve(),
+            resolve({
+                dedupe: ['svelte']
+            }),
             commonjs(),
             json()
         ],
         external: Object.keys(pkg.dependencies).concat(
             require('module').builtinModules || Object.keys(process.binding('natives'))
         ),
+        preserveEntrySignatures: 'strict',
         onwarn,
     },
 
@@ -105,6 +112,7 @@ export default {
             commonjs(),
             !dev && terser()
         ],
+        preserveEntrySignatures: false,
         onwarn,
     },
 };