doc: update pull request template URL layout

Putting the link for the commit guideline at the bottom of the pull request template makes it a bit confusing when opening a pull request as the markdown does not render. Not everyone is familiar with the syntax used there and it is easy to mess it up (for example, by not leaving a blank line above it). It is more natural and useful if the URL is included inline.

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc