Replace custom rem hugo shortcode template with HTML comment. HTML comments are now
allowed in Hugo without enabling unsafe HTML parsing.
Improve the text in the comment: Remove unnecessary quoting, and avoid impression that
make backenddocs has to be run and results committed, since we have a lint check which
will then report error because we want to prevent manual changes in autogenerated sections.
Disable the markdownlint rule line-length on the autogenerated marker line.
Make the autogenerated marker detection a bit more robust.
See #8942 for more details.
Before this change the pikpak backend changed the global
--multi-thread-streams flag which wasn't desirable.
Now the machinery is in place to use the NoMultiThreading feature flag
instead.
Fixes#6915
