Undocumented UI assumption

If I'm reading the code correctly the extension expects the UI to put the content in an article tag with class="doc". This should be documented in the readme under "UI Assumptions".

https://gitlab.com/antora/antora-lunr-extension/-/blob/a70608276cf4287111f20a0cb5228df030d67f46/lib/generate-index.js#L111

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