Remove details-summary construct from changelog template

[redeboer]

Currently, the release drafter summarises all pull requests with details-summary syntax. This looks fancy, but results in duplicate information: one could just click the PR link for more info. In addition:

• The info under the folded details not searchable (which was the main intend).
• There is duplicate info (no SSoT). It would be better to just update the details in the description of each listed PR, if there is any info.
• Info under <summary> does not support Markdown

Another motivation: RTD recently started removing PR preview build files, which means that they cannot be used as permalinks anymore (which is often done as links to notebook previews in PR descriptions). One should instead switch to links to the specific version pages on RTD (e.g. ampform.readthedocs.io/en/0.14.0), which can only be done by updating the PR descriptions after the version has been released.

[redeboer]

Perhaps this can be complemented with:
https://github-activity.readthedocs.io/en/latest/