Docs: mention that Docusaurus needs `trailingSlash: true` (default) to work

We are documenting Docusaurus at https://docs.readthedocs.com/platform/latest/intro/docusaurus.html

We should mention that `trailingSlash: true` is required to work without issues. When it's `false`, the user needs to define a redirect: https://github.com/readthedocs/readthedocs.org/pull/12323