Add alt tags to images for a11y (#529)

- Add imageAlt to GridBlock
- Set alt equal to `user.caption` for mapped users
- Set alt equal to `post.author` for blog posts
- Update documentation

lib/core/nav/HeaderNav.js

@@ -217,6 +217,7 @@ class HeaderNav extends React.Component {
                 <img
                   className="logo"
                   src={this.props.baseUrl + siteConfig.headerIcon}
+                  alt={siteConfig.title}
                 />
               )}
               {!this.props.config.disableHeaderTitle && (