feat(docs): add docs about how to deploy Vikunja in a subdirectory

docs/content/doc/setup/subdirectory.md

@@ -0,0 +1,39 @@
+---
+title: "Running Vikunja in a subdirectory"
+date: 2022-09-23T12:15:04+02:00
+draft: false
+menu:
+  sidebar:
+    parent: "setup"
+---
+
+# Running Vikunja in a subdirectory
+
+Running Vikunja in a subdirectory is not supported out of the box.
+However, you can still run it in a subdirectory but need to build the frontend yourself.
+
+## Frontend
+
+First, make sure you're able to build the frontend from source.
+Check [the guide about building from source]({{< ref "build-from-source.md">}}#frontend) about that.
+
+Then, run 
+
+```
+pnpm vite build --base=/SUBPATH
+pnpm workbox copyLibraries dist/
+```
+
+Where `SUBPATH` is the subdirectory you want to run Vikunja on.
+
+Once you have the build files you can deploy them as usual.
+Note that when deploying in docker you'll need to put the files in a web container yourself, you 
+can't use the `Dockerfile` in the repo without modifications.
+
+## API
+
+If you're not using a reverse proxy you're good to go.
+Simply configure the api url in the frontend as you normally would.
+
+If you're using a reverse proxy you'll need to adjust the paths so that the api is available at `/SUBPATH/api/v1`.
+You can check if everything is working correctly by opening `/SUBPATH/api/v1/info` in a browser.