Summary:
Several blog and sharing improvements are included in this update.
* A larger default preview image is used when a blog post or doc is shared on social media as recommended by [best practices](https://developers.facebook.com/docs/sharing/best-practices#images).
* Follow other Facebook Sharing best practices such as associating the site with a Facebook Page (React) and a Page Admin (121800083 is my fbid, and I am an admin for the React page).
* If the shared blog post contains a hero image, use it as a share preview image.
* Use actual doc content instead of generic site description when sharing docs.
* Update existing hero images to use the larger recommended size.
* Add titles to each author's byline.
* Add author's avatar to their byline.
* Add Twitter Card support.
* Link to individual blog posts from the title and hero image in the blog index.
Old metadata tags (blog post):
```
<meta property="og:title" content="Toward Better Documentation – React Native | A framework for building native
Closes https://github.com/facebook/react-native/pull/9648
Differential Revision: D3790332
Pulled By: hramos
fbshipit-source-id: 4f284c6440482df8a42a2b8467ccf56b1fd725d1
Currently all the pages have the same title, making it hard to
tell which page is while when user has multiple tabs opened.
This adds 'title' to Site so a page title can be passed in and
prepended to the default title
Brian Vaughn
Tim Yung
Héctor Ramos
Sokovikov
Mu-An Chiou
Christopher Chedeau