Add --abs-proxy-base-path for when code-server is not at the root (#6958)

docs/guide.md

@@ -20,6 +20,7 @@
   - [Proxying to a Vue app](#proxying-to-a-vue-app)
   - [Proxying to an Angular app](#proxying-to-an-angular-app)
   - [Proxying to a Svelte app](#proxying-to-a-svelte-app)
+  - [Prefixing `/absproxy/<port>` with a path](#prefixing-absproxyport-with-a-path)
 
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@@ -432,3 +433,12 @@ const config = {
 3. Access app at `<code-server-root>/absproxy/5173/` e.g. `http://localhost:8080/absproxy/5173/
 
 For additional context, see [this Github Issue](https://github.com/sveltejs/kit/issues/2958)
+
+### Prefixing `/absproxy/<port>` with a path
+
+This is a case where you need to serve an application via `absproxy` as explained above while serving `codeserver` itself from a path other than the root in your domain.
+
+For example: `http://my-code-server.com/user/123/workspace/my-app`. To achieve this result:
+
+1. Start code server with the switch `--abs-proxy-base-path=/user/123/workspace`
+2. Follow one of the instructions above for your framework.