Publish towncrier draft on "latest" docs

[vytas7]

At the time of this writing, we check that Towncrier fragments render correctly in the CI, but we are not continuously publishing that preview in the unstable version of our docs.

@vytas7 was talking to @webknjaz at EuroPython2024, and @webknjaz pointed out that he had written a tool for that purpose: https://github.com/sphinx-contrib/sphinxcontrib-towncrier. We should evaluate if this tool fits our docs infrastructure, and start publishing draft release notes.

[webknjaz]

I sent this to you on matrix, but here's a public copy of some change log-related examples/improvements links:

• https://yarl.aio-libs.org/en/latest/changes/
• https://yarl.aio-libs.org/en/latest/contributing/guidelines/#adding-change-notes-with-your-prs
• https://github.com/cherrypy/cheroot/blob/1ff20b1/towncrier.toml
• https://github.com/cherrypy/cheroot/blob/1ff20b1/docs/changelog-fragments.d/.towncrier-template.rst.j2
• https://github.com/cherrypy/cheroot/blob/1ff20b1/.pre-commit-config.yaml#L11-L57
• https://github.com/cherrypy/cheroot/blob/1ff20b1/docs/history.rst?plain=1
• https://github.com/cherrypy/cheroot/blob/1ff20b1/docs/conf.py#L12-L20
• https://github.com/cherrypy/cheroot/blob/1ff20b1/docs/conf.py#L40
• https://github.com/cherrypy/cheroot/blob/1ff20b1/docs/conf.py#L52-L54
• https://github.com/cherrypy/cheroot/blob/1ff20b1/docs/conf.py#L156-L162
• 📝 Replace GH/PyPI extlinks w/ sphinx-issues pytest-dev/pytest#12522