Fix @externalDocumentation rendering

[msosnicki]

Background

Currently the key and value from the @externalDocumentation trait is taken in the wrong order, leading to rendering of the following:

<a href="External docs">https://external.docs/structureTrait</a>

• What do these changes do?
• Why are they important?

Testing

• How did you test these changes? Added unit test

Links

• Links to additional context, if necessary
• Issue #, if applicable (see here for a list of keywords to use for linking issues)

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[msosnicki]

Made these just for demonstration purposes, can revert as this is not related to this PR. Uncommenting these lines will make the test fail, even though there is

    @externalDocumentation(
        "External docs": "https://external.docs/structureTrait#fieldA"
    )
    fieldA: String

To make it work, it has to contain a documentation section:

    /// some other docs
    @externalDocumentation(
        "External docs": "https://external.docs/structureTrait#fieldA"
    )
    fieldA: String

I believe this is because how the interceptors are implemented, the ExternalDocsInterceptor has

    @Override
    public Class<JavaDocSection> sectionType() {
        return JavaDocSection.class;
    }

but this condition does not hold in the failing case.

I can raise an issue if you also feel that this is a problem