chore: enhance doc-changelog migration guide

[klmcadams]

Updated the doc-changelog migration guide to clarify setting up the release notes in the CHANGELOG.md file versus the documentation (changelog.rst). Although a couple of the steps are repeated in the "release notes in your documentation" and "release notes in CHANGELOG.md" sections, I think it's clearer.

Previously, you would have to set up everything for the CHANGELOG.md configuration, and then change most of what you had set up if you want to include it in your documentation instead.

[ansys-reviewer-bot[bot]]

Thanks for opening a Pull Request. If you want to perform a review write a comment saying:

@ansys-reviewer-bot review

[klmcadams]

@ansys/pyansys-core do you guys prefer having a space between the steps or no space?

Space:

No space:

[RobPasMue]

@ansys/pyansys-core do you guys prefer having a space between the steps or no space?

Space:

No space:

This is related to: https://github.com/ansys/actions/pull/499#discussion_r1630765632