Cleanup feature proposal template

What does this MR do?

Changes the recommended template for feature proposals.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

I've been using the template for a while, and find that putting so many words into the h2 makes me think there's actual content there, until I realize I'm just reading the template text (again). I think we can get away with just putting Description as the header, and putting the suggested content for that section as plain text below for someone to replace.

What are the relevant issue numbers?

Screenshots (if relevant)

/cc @JobV