Automate adding release notes with a script

[cwickham]

From quarto-dev/quarto-web#1250

Updating the release notes will currently require a manual step at release:

We would have to add the following tasks to the release checklist:

1. Copy changelog from old release to _changelog.md in docs/download/changelog/1.x

2. Copy docs/download/changelog/1.*/index.qmd and increment versions

3.Add new item to download-older listing in docs/download/index.qmd

4. Increment version in aliases of:

   • docs/download/release.qmd
   • docs/download/prerelease.qmd

As per @cderv's suggestion, we could write a script so these instructions are replaced by, e.g. :

quarto run update-release-notes.R