Add redcarpet gem for Markdown support.

[reitermarkus]

We use --markup markdown but don't have redcarpet specified as a dependency, meaning docs are not actually being generated using Markdown, or at least not Github Flavored Markdown.

[dduugg]

Did you test this? I think you'll need --markup-provider=redcarpet, in https://github.com/Homebrew/brew/blob/master/Library/Homebrew/.yardopts well, no?

[reitermarkus]

According to https://github.com/lsegal/yard/blob/e833aac7a01510245dd4ae1d1d18b046c8293c2d/docs/WhatsNew.md?plain=1#L542-L547, simply adding it to the Gemfile is enough as it is now the default behaviour for --markup markdown. Also tested this locally.

[Bo98]

Do you have an example page so we can compare the effect this has before & after merge?

[reitermarkus]

It doesn't have any effect in the current docs, because they mostly use <pre> instead of Markdown ```.

[MikeMcQuaid]

Makes sense, thanks @reitermarkus!