docs: add clarification about Analytics SDK

[tafelnl]

Your documentation mentions sending push notifications from the Firebase dashboard.

However, I think this is kind of misleading. Sending out notifications this way, will only work for test devices on a debug app. For 'normal' devices, this doesn't work, unless you have included the Firebase Analytics SDK. This is not mentioned anywhere.

I've made a proposal for a possible clarification about this.

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BTW, it's not very clear from the Google docs that this is the case. But it definitely is. You can test it out for yourself. The Google docs only say it pretty ambiguously:

For an optimal experience using this product, add the Firebase SDK for Google Analytics.

Edit: I found a little bit more clear explanation in the docs (still a little vague if you ask me):

Note: For an optimal experience with FCM, we strongly recommend enabling Google Analytics in your project. Google Analytics is a requirement for FCM's message delivery reporting.

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Name	Status	Preview	Updated
capacitor-docs	✅ Ready (Inspect)	Visit Preview	Jan 16, 2023 at 10:52AM (UTC)