docs(push-notifications-firebase): Xcode is required

[rbalet]

Description

I've added it manually, through vsCode, and it didn't worked.

So I figured adding a little warning may be helpful

[vercel]

@rbalet is attempting to deploy a commit to the Ionic Team on Vercel.

A member of the Team first needs to authorize it.

[jcesarmobile]

Well, it says "open Xcode"

then "and move the .plist file into your Xcode project as instructed by Firebase, ensuring to add it to all targets."
Firebase instructions also use a similar wording.

then a screenshot of Xcode with the file.

I don't think more hints are needed, if you added it from VSCode you didn't move it to your "Xcode project" but to just your project, and you didn't add the file "to all targets" since that's not possible from VSCode.

In any case, we could change the "You'll then want to" to something like "You'll then have to", since it's not really an option to not do it.

[rbalet]

@jcesarmobile I give you right, and maybe that is just me (and the couple of other thread I saw on the internet).

I've updated the text in favor of the have to with an emphasis on it.

Maybe it would help people like me that read through.
You can always close it if you do not find the use of it.

Thx