Add tidy check for missing `{# #}` tag in rustdoc templates

[GuillaumeGomez]

To make the generated HTML size smaller, we strip whitespace characters as much as possible in our jinja templates, using {# #} (jinja comments) to force the trimming between HTML items.

However, we often forget to put {# #} between HTML items (like in #120818 most recently). A tidy check would fix this issue.