Site editing guide, link button, remove slug, forward latest to root, and custom 404.html template

@jkridner With this we have the following things added to our docs site,

1. Copy link button next to every heading, figure, and table. Also changed the feedback button icon as per the user's suggestion.
2. Added site editing guide for new users to easily contribute to our documentation site.
3. Removed /latest slug and forwarded all /latest pages to root thus no broken links.
4. Added custom 404.html template page with a link on the page to open feedback modal to get feedback from users.