Document that 'use_directory_urls' is mandatory

Without it:

hrefs mistakenly have .html on their path in some places, leading to 404s

clicking on the platform field [here] (https://repo.kernelpanicsoft.net/docs/Archie/api/common/net.kernelpanicsoft.archie/-a-platform/index.html#platform) yields error 404 because it should link to platform/index.html not platform.html