Ignore prerelease tags when generating changelogs, and add some changelog API examples

What does this MR do?

When generating a changelog without specifying the to argument, we use the tag of the last release. If a project has any prerelease tags, such as GitLab itself, we would end up using such a tag. But this is undesired, as changelogs are usually only updated for stable releases (as is the case for GitLab for example).

To solve this, we no longer consider prerelease tags. This means that in the above example we'd instead use the tag of the last stable release, as one would expect.

In addition, this MR adds some examples to the changelog API documentation, hopefully making it easier to understand how to use this API.

This fixes gitlab-com/gl-infra/delivery#1574 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content