Auto-publish nightly & stable releases on GitHub Discussions

[oberstet]

Feature Description

one the realease.yml workflow has run after a merge of a PR to master, eg

https://github.com/crossbario/txaio/actions/runs/19916859381

it'll produce and publish a Nightly Release on GitHub Releases, eg

https://github.com/crossbario/txaio/releases/tag/master-202512040342

but it should also publish an announcement on discussions:

https://github.com/crossbario/txaio/discussions

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we have that stuff already for https://github.com/crossbario/autobahn-python and we should mimick/copy-paste that to txaio!