[docs-only] add changelog template for csv changelog output

[dragotin]

To be able to create "the spreadsheet", we need the changelog in CSV format.

Somebody with drone knowledge should add the following command line to the file .drone.star (it seems):

calens -o dist/changelog.csv -t changelog/changelog-csv.tmpl 

to be executed when releasing.

[update-docs[bot]]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

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