This PR is now based on #921. I went through the ~200 issues|PRs|commits and I created ~75 news fragments.

I changed the categories/types to add doc and feature, and I removed change.

I found some limitations in towncrier:

• It's not possible to assign multiple PRs|issues to a single fragment. In order to get multiple refs in one item, the fragment file needs to be duplicated with different filenames. The content must be exactly the same (including line-endings). I think the duplication is acceptable until they improve it in towncrier.
• It's possible to have fragments with commit refs, instead of PRs|issues. For instance, g758b9e47.deprecation.rst. However, it's not possible to have them linked. This is because configuration option issue_format supports a simple replacement of variable {issue}; it does not support complex logic to select the format depending on the ID/prefix. Therefore, I decided to use +758b9e47.deprecation.rst, which shows those items without any ID/ref.
• Since the template or the issue_format cannot be overriden in CLI, it's not straightforward to support two outputs (one for GitHub Releases without sphinx features, and another one to be included in sphinx). Even if we used markdown only, we would still need to maintain two configuration files. Therefore, I propose to stop adding release notes to the GitHub Releases, and instead just add a link to the corresponding section in the docs. This PR adds labels before each subsection so we can use https://vunit.github.io/release_notes.html#release-4-6-0.

@LarsAsplund @GlenNicholls if you are good with the implementation in #921 and the news fragments introduced in this PR, we can introduce towncrier already and use it to handle the remaining PRs before releasing v4.7.0. Actually, we can merge #921 and #918 and keep changing the fragments until we decide to release v4.7.0.