[BUG] Improve and reference Changelog docs in PR docs

[danielrobbins]

We have Changelog documentation here:

https://docs.saltproject.io/en/latest/topics/development/changelog.html

However, this documentation is not integrated or referenced in our documentation on how to create PRs:

https://docs.saltproject.io/en/latest/topics/development/contributing.html

The "Changelog" section appears BEFORE the Contributing section, but should likely be placed INSIDE the Contributing section. It is a key part of our PR process, and we have several community contributors who write their own Changelog entries.

Thus, it should be integrated into the steps listed in contributing.html.

[OrangeDog]

It is referenced in the PR template at least.