Libmarkdown to string

[CarwynNelson]

Related to #17128

This PR changes Document::render_to_html and Document::render_to_inline_html in LibMarkdown to return an ErrorOr<String> instead of a DeprecatedString, as well as changes to all the places that consume this function.

Almost all of the consumers of these functions do not have error propagation, so in the interests of keeping the diff small (and not getting yak baited) I have decided to use release_allocated_value_but_fixme_should_propagate_errors().