Create documentation for new API!

[julianlumets]

Hey! I cannot find a documentation suiting for the new api ( i am using angular fire 7.4.1) ? It would be really nice if the doc gets updated or you could tell me where i can find all the new methods and so on.... thank you!

[google-oss-bot]

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[mystiquewolf]

Who sane person releases new API without documentation? Insanity!

[jamesdaniels]

The new API is largely the same as Firebase JS SDK as we just wrap it. This is documented on the firebase site. WIP for the new reference-style docs are here #3236