doc: clarify using fleet api has lots of drawbacks

[JakobLichterfeld]

Thank you. Since you were the first to stumble, you won't be the last. So I am always open to improvements and I understand your point.
This was the point that stopped me from merging or implementing the Fleet API in the first place, as there are only drawbacks for 99.9% of users.
Using the fleet API is not recommended and has not been recommended in any issue or discussion.
The name official indicates something else, I got this point.

Originally posted by @JakobLichterfeld in #4081 (comment)

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[JakobLichterfeld]

@emilecantin: please review: https://deploy-preview-4173--teslamate.netlify.app/docs/guides/api

[emilecantin]

@JakobLichterfeld Yeah, this is much clearer, thanks!

[jlestel]

Great job 👏