Issue #687 - Reorganise NEWS file before release

[nikosbosse]

Description

This PR closes #687. The PR reorganises the news items for the current major package update.

Checklist

• [x] My PR is based on a package issue and I have explicitly linked it.
• [x] I have included the target issue or issues in the PR title as follows: issue-number: PR title
• [x] I have tested my changes locally.
• [ ] ~I have added or updated unit tests where necessary.~
• [ ] ~I have updated the documentation if required.~
• [x] I have built the package locally and run rebuilt docs using roxygen2.
• [ x My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• [x] I have added a news item linked to this PR.
• [ ] I have reviewed CI checks for this PR and addressed them as far as I am able.

[nikosbosse]

Latest update:

• I added an introductory section that motivates the current changes and gives a short overview of what we did
• I added an explanation for why we dropped support for the interval format