Standarize the changelog section of PR descriptions

[evanlinjin]

I propose standardizing the changelog section of PR descriptions so it's easier for scripts to scrape them for releases.

For example:

• Include relevant crate[s] per change (since we have a changelog per crate).
• Standardizing formatting - headings, bullet points, wrap in block, etc (can add this to the PR template).

[oleonardolima]

I'd also suggest adding another step on the github-merge script, in order to prompt the maintainer merging the PR to check and attest that it currently has a valid changelog info.