Explain the db providers necessity in slim images

[christophelec]

related: #35255

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This PR adds a small clarification in the documentation, regarding the fact that while extra database clients are installed on a slim images, databases providers are not (and are not part of the default providers installed)

[boring-cyborg]

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[potiuk]

Works for me if it works for you :)

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.