Updates Push Notifications with Firebase Guide

[javebratt]

Hey y'all, this PR pushes a few updates on the push notifications article:

• Most of the references where to Ionic 4. Changed it to reference to Ionic 5.
• The first part was prompting the user to install the old ionic CLI package. Changed to install the @ionic/cli.
• After we add the code to the home.page.ts there was no notice that the user needed to run a new build and npx cap copy to copy web assets to the android application, so that code wasn't getting executed.
• The instructions to add a new android app in Firebase worked only for new projects, existing projects didn't have that Icon so added some more details and a picture to show where to create it.

[javebratt]

There's also a broken link here:

Building and deploying iOS and Android applications using Capacitor requires a bit of setup. Please follow the instructions to install the necessary Capacitor dependencies here before continuing.

I don't know how to fix it, because it works properly on localhost, but deployed it's adding the word 'guides' to the URL.

[dotNetkow]

There's also a broken link here:

Building and deploying iOS and Android applications using Capacitor requires a bit of setup. Please follow the instructions to install the necessary Capacitor dependencies here before continuing.

I don't know how to fix it, because it works properly on localhost, but deployed it's adding the word 'guides' to the URL.

You can ignore this, as it's rendering the link correctly when deployed. I think the difference is the path generated locally vs when deployed. Annoying but OK for now. good call out though :)