Improve `invoke generate-release-notes` to place the towncrier generated content in the appropriate file automatically

[cmsirbu]

As a repository maintainer, I would like to have a simpler release process, in this case taking the output of towncrier and placing it in the correct release notes file:

• For patch releases, insert it at the right marker in the existing markdown file
• For major/minor releases, generate the new release notes file from a template with the appropriate filename derived from the pyproject version first, then insert the release notes