Not sure if this can be directly put into main or I should target the new_web_theme branch.

This PR refactors the changelogs a bit. In particular, it avoids multiple top-level headings in one file, which should not have happened and is assumed by pydata-sphinx-theme when designing its heading styles. Also in this way the toctree will not show for instance 1.3.2, 1.3.1, 1.3.0, ..., but instead show only 1.3 which I think is desired.

Previous	Current
The micro versions are all shown because they are all top-level headings. I do not see a simple way to avoid this in sphinx: using :titlesonly: or :maxdepth: 1 would not work.	We have top-level headings as minor version only. This may not look like much of an improvement with the current theme, but since pydata-sphinx-theme has the left sidebar that shows the global toctree, this change would make the navigation much simpler.
There are multiple top-level headings for a single page. This is not a problem with the current theme but pydata-sphinx-theme assumes one top-level heading per page when designing its heading styles, so this would become an issue.	There is only one top-level heading. Consequently the toggle menu in the current theme may provide less information, but this is not a problem for pydata-sphinx-theme since it has full pagetoc in the secondary sidebar.

Other changes

Since we have a single top-level heading now, I moved some "global" information (i.e., not specific to some micro version) from under the first micro version (e.g. 1.3.0) to under the top-level heading (e.g. 1.3). These include: the changelog legend, the link to the release highlight example, and the contributor list. Also note that the titles of the changelog legend and the contributor list are made into .. rubric:: to avoid them from appearing in the toctree. Rubrics in the current theme may not look outstanding enough, but with pydata-sphinx-theme they definitely look good.