Add rouge-ruby Comment.Doc style support

What does this MR do and why?

Rouge Ruby, which GitLab uses as its highlighter, has introduced a Comment::Doc comment type many years ago, which uses class cd for highlight.

However, for a long time, neither Rouge nor any of its applicants added any style for this rather new type of comment, I believe it is mainly because only highlighted document comments using php, haskell, rust, r and lasso have it equipped.

I recently found this bug when the PHP doc did not work in the file view of our GitLab instance, earlier today I also submitted a PR fixing Rouge Dingus's style.

Screenshots or screen recordings

Wrong Highlight Behaviors	Expected Behaviors / Result of this PR

How to set up and validate locally

Write the following PHP code:

<?php
class Test
{
    /**
     * This is a test.
     * @param int $base The base number.
     * @param int $step The step number to add to the base number.
     * @return int The sum of base and step numbers.
     */
    public function testing(int $base = 0, int $step = 1) :int
    {
        return $base + $step;
    }
}

The php doc section should look like this:

    /**
     * This is a test.
     * @param int $base The base number.
     * @param int $step The step number to add to the base number.
     * @return int The sum of base and step numbers.
     */

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.