feat(v2): improve seo (#1588)

* feat(v2): improve seo

- Add doc specific seo information
- Add twitter meta tags to seo in theme Layout

* Cannot use svg in SEO image

* Add docs about SEO

* add site default image for seo

* Resolve PR discussion

* Add `image` to config optional fields

* Use theme config instead of root config for image, rename in front matter

* Use absolute url for image (wont work on preview)

* update docs for frontmatter field for seo

* refactor

* pages seo

* fix

* nits again'

website/src/pages/feedback/index.js

@@ -23,7 +23,7 @@ function Feedback() {
   }, []);
 
   return (
-    <Layout>
+    <Layout permalink={'/feedback'} description={'Docusaurus 2 Feedback page'}>
       <div className="container margin-vert--xl" data-canny />
     </Layout>
   );