Don't insert images into and preview from `pages` dir when in `antora` mode

[m1027]

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I'd like to ask for further tweaks when in antora mode:

In 3.1.10, images are still detected and previewed from the pages directory, although antora wouldn't accept this later on. So, users in antora mode may get misled by the (working) preview.

Especially because...images pasted from the clipboard get inserted into the pages directory and not into the images directory. So, users in antora mode need to remember moving them manually afterwards. It would be fantastic to honor the antora location for the paste action.

Originally posted by @m1027 in https://github.com/asciidoctor/asciidoctor-vscode/issues/434#issuecomment-1826862716