docs: add xref support for adoc file sourced from javadoc

[aldettinger]

More context with this commit.

With a @configitem as below:

/**
     * If {@code true} all {@code camel-main} features are enabled; otherwise no {@code camel-main} features are
     * enabled. See described the
     * <a href="https://camel.apache.org/camel-quarkus/latest/user-guide/bootstrap.html#_camel_main">Bootstrap</a>
     * section of Camel Quarkus documentation for more details.
     */
    @ConfigItem(defaultValue = "true")
    public boolean enabled;

The generated adoc documentation in docs/modules/ROOT/pages/reference/extensions/core.adoc will contain:

link:https://camel.apache.org/camel-quarkus/latest/user-guide/bootstrap.html#_camel_main[Bootstrap]

while an xref as below is expected:

xref:user-guide/bootstrap.adoc[Bootstrap].