Draft: Clarify intermediate cert file options

What does this MR do?

While working with a customer, I found that this section of our self-signed cert doc was a bit misleading, particularly around placing all certs in a chain in one file.

In reality, while some images may allow users to place all certs in one file, others require each to be in a separate file, and still others may allow either option.

In order to elaborate on this, I have moved the confusing "note" to its own section, and added more context. My goal is to clarify that there are multiple ways to approach this, depending on use case. My goal is to make the document more clear, while accounting for more use cases. Having the section written as it is now would have deflected at least one ticket, and ought to help deflect more in the future, which in my opinion is worth the change!

Related issues

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