Img closing tags removed from XHTML tags

Because `img` tags are empty elements, they should not have any inner content. SUSHI tries to minify the XHTML and the minifier removes the closing `` tag. Example from Zulip in FSH Online [here](https://fshschool.org/FSHOnline/#/share/35ZHcND). While this is not valid XHTML, it is probably okay that SUSHI does this, but we might be able to better warn the users to alert them to what happened.

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/Weird.20non-rendering.20of.20xhtml.20in.20generated.20resource/near/275690452

Reproduce the Zulip example through the linked FSH Online share and inspect the XHTML minification path responsible for removing the closing img tag. Confirm the resulting output and determine a clear warning for users; done means the behavior is intentionally communicated without breaking generated resources.

Written by the indexing model from the issue text.