Supress warning issued for redirect pages

[SeekingMeaning]

This is a 🐛 bug fix.

Summary

This updates lib/jekyll/commands/doctor.rb to skip redirect pages (from the jekyll-redirect-from plugin) when checking for permalink conflicts

Context

Closes #8346

[ashmaroli]

I think having a separate :allow_used_permalink? method should be safe.
If there are users facing this issue with another plugin, we can ask them to write a local plugin that redefines the said :allow_used_permalink? method to suit their need..

def allow_used_permalink?(item)
  defined?(JekyllRedirectFrom) && item.is_a?(JekyllRedirectFrom::RedirectPage)
end

Additionally, this change definitely needs a test. You can add another Cucumber scenario like before.

[SeekingMeaning]

I think having a separate :allow_used_permalink? method should be safe.

Just making sure, where should this method be added?

[ashmaroli]

where should this method be added?

It can be a private class method in Jekyll::Commands::Doctor

[ashmaroli]

Thank you for the quick turnarounds @SeekingMeaning
I would like the cucumber scenario to more resemble a real-world use-case.

redirect_from ["redirect.html", "redirect.html"] doesn't seem clear enough to me.

If you're able to change it, great! Otherwise, I shall update this branch when I can come up with a nice scenario.

[ashmaroli]

Thank you @SeekingMeaning
@jekyllbot: merge +minor