System/Trust - explain a bit around intermediate certs in certificates.rst

[AdSchellevis]

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• I have read the contributing guide lines at https://github.com/opnsense/docs/blob/master/CONTRIBUTING.md
• I have searched the existing issues and I am convinced that mine is new.

Is your feature request related to a problem? Please describe.

Triggered by opnsense/core#9117, it might be good to explain a bit about the trust chain and the importance of intermediate certs in some cases. Although it's possible to deploy them on OPNsense, there are also some downsides as we noticed a couple of years ago with the Let's Encrypt issue.

Describe the solution you like

Some small basic explanation about trust and which defaults we ship.

Describe alternatives you considered

none

Additional context

[AdSchellevis]

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