Updated dead links (#9853)

- Updated dead link "/audit-with-lighthouse/" to "/docs/audit-with-lighthouse/"
- Updated dead link "/add-a-manifest-file/" to "/docs/add-a-manifest-file/"

<!--
  Q. Which branch should I use for my pull request?
  A. Use `master` branch (probably).

  Q. Which branch if my change is a bug fix for Gatsby v1?
  A. In this case, you should use the `v1` branch

  Q. Which branch if I'm still not sure?
  A. Use `master` branch. Ask in the PR if you're not sure and a Gatsby maintainer will be happy to help :)

  Note: We will only accept bug fixes for Gatsby v1. New features should be added to Gatsby v2.

  Learn more about contributing: https://www.gatsbyjs.org/docs/how-to-contribute/
-->

Author: MrRobotjs

docs/docs/add-offline-support-with-a-service-worker.md

@@ -2,9 +2,9 @@
 title: Add Offline Support with a Service Worker
 ---
 
-If you've run an [audit with Lighthouse](/audit-with-lighthouse/), you may have noticed a lackluster score in the "Progressive Web App" category. Let's address how you can improve that score.
+If you've run an [audit with Lighthouse](/docs/audit-with-lighthouse/), you may have noticed a lackluster score in the "Progressive Web App" category. Let's address how you can improve that score.
 
-1.  You can [add a manifest file](/add-a-manifest-file/). Ensure that the manifest plugin is listed _before_ the offline plugin so that the offline plugin can cache the created `manifest.webmanifest`.
+1.  You can [add a manifest file](/docs/add-a-manifest-file/). Ensure that the manifest plugin is listed _before_ the offline plugin so that the offline plugin can cache the created `manifest.webmanifest`.
 2.  You can also add offline support, since another requirement for a website to qualify as a PWA is the use of a [service worker](https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API). [Gatsby's offline plugin](/packages/gatsby-plugin-offline/) makes a Gatsby site work offline--and makes it more resistant to bad network conditions--by creating a service worker for your site.
 
 ### What is a service worker