Improvements to 404 page

Let's provide a better UX when our users land in a 404 page:

• Add a back button https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/312
• Add a search bar taking the query to /search/ => https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/345
• Update text, improve image => https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/345
• Show results as you type like on home page, but consider that it is not a comprehensive list of the results - should we clarify that in the UI in both places? => https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/345
• Consider using same image as GitLab the application uses (replace current image) => https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/345
• Display search results from auto-search based on the URL (similar to this} => https://gitlab.com/gitlab-com/gitlab-docs/issues/265 // https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/350