[8.15] Add section about Env Provider availability for Fleet-managed agent (backport #1285)

[mergify]

[Based on a Slack discussion] The Env Provider is available for use in Fleet-managed Elastic Agent (i.e., not only for standalone Elastic Agent), so this updates the Fleet-managed agent docs with a short section about that.

Closes: #1283

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This is an automatic backport of pull request #1285 done by [Mergify](https://mergify.com).

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