misc(v2): change blog front matter to snake_case (#1989)

* misc(v2): change blog front matter to snake_case

* Fix blog image url

* Add docs for change

* More migration docs

website/docs/blog.md

@@ -26,10 +26,11 @@ For example, at `my-website/blog/2019-09-05-hello-docusaurus-v2.md`:
 ```yml
 ---
 title: Welcome Docusaurus v2
-author: Dattatreya Tripathy
-authorTitle: Contributor of Docusaurus 2
-authorURL: https://github.com/dt97
-authorTwitter: CuriousDT
+author: Joel Marcey
+author_title: Co-creator of Docusaurus 1
+author_url: https://github.com/JoelMarcey
+author_image_url: https://graph.facebook.com/611217057/picture/?height=200&width=200
+authorURL: https://github.com/JoelMarcey
 tags: [hello, docusaurus-v2]
 ---
 Welcome to this blog. This blog is created with [**Docusaurus 2 alpha**](https://v2.docusaurus.io/).
@@ -46,8 +47,9 @@ A whole bunch of exploration to follow.
 The only required field is `title`; however, we provide options to add author information to your blog post as well along with other options.
 
 - `author` - The author name to be displayed.
-- `authorURL` - The URL that the author's name will be linked to. This could be a GitHub, Twitter, Facebook account URL, etc.
-- `authorImageURL` - The URL to the author's image. (Note: If you use both `authorFBID` and `authorImageURL`, `authorFBID` will take precedence. Don't include `authorFBID` if you want `authorImageURL` to appear.)
+- `author_url` - The URL that the author's name will be linked to. This could be a GitHub, Twitter, Facebook profile URL, etc.
+- `author_image_url` - The URL to the author's thumbnail image.
+- `author_title` - A description of the author.
 - `title` - The blog post title.
 - `tags` - A list of strings to tag to your post.