[Docs] Add prerequisite target to "build a sample app" (#1907)

* Add prerequisite target to "build a sample app"

Hi,

While trying to get a simple app running on Yew, I had the target wasm32-unknown-unknown missing so my project couldn't work.

I managed to find the missing target (by searching from the error message I was getting) on the same "Getting started" documentation, but later than the "build a sample app" page I was on.

It is my opinion that this can be needed during the "build a sample app" step, earlier than the "Project setup" step.
Another option could be to reference the "Project setup" from "build a sample app", in order to troubleshoot why the sample app is not working.

This is but a proposal and if you think this is not needed, please close this PR.

* Update website/versioned_docs/version-0.18.0/getting-started/build-a-sample-app.md

Co-authored-by: Teymour Aldridge <42674621+teymour-aldridge@users.noreply.github.com>

Co-authored-by: Teymour Aldridge <42674621+teymour-aldridge@users.noreply.github.com>

website/versioned_docs/version-0.18.0/getting-started/build-a-sample-app.md

@@ -107,6 +107,13 @@ If you haven't already, install [Trunk](https://github.com/thedodd/trunk):
 cargo install trunk wasm-bindgen-cli
 ```
 
+If you haven't already installed it, you need to add the `wasm32-unknown-unknown` target. 
+To install this with Rustup:
+
+```bash
+rustup target add wasm32-unknown-unknown
+```
+
 Now all you have to do is run the following:
 
 ```bash