Add checkout Action so the template can be found

[lee-dohm]

I found that creating an issue with a custom template failed because the file couldn't be found locally. Adding the checkout Action solved the problem for me, so I figured I'd submit a change for the docs.

I'm not sure if this is because I was working in a private repo or if it needs to be used in all cases 🤷‍♂

[JasonEtco]

You're not the first to encounter this, but I didn't realize it wasn't in the docs! This is a super helpful addition, thanks @lee-dohm! ✨