Use consistent path for downloadable satre.xlsx spreadsheet

[manics]

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:arrow_heading_up: Summary

Currently we use the Sphinx :download: role to make the satre.xlsx spreadsheet available, but this results in a random hash being included in the published path. This is annoying since we can't link directly to the spreadsheet.

This PR is a workaround so that satre.xlsx is always in the base directory, without a prefix, so in future we can use https://satre-specification.readthedocs.io/en/stable/satre.xlsx instead of https://satre-specification.readthedocs.io/en/stable/_downloads/<random-hash-which-changes-every-release>/satre.xlsx

:closed_umbrella: Related issues

:raising_hand: Acknowledging contributors

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[manics]

The linkcheck step is still failing.... without indicating what the error is 🙁. It runs fine locally.

[manics]

Another benefit of a consistent URL is we could automatically compare the last stable (tagged) satre.xlsx with the version from a PR, and use https://github.com/simonw/csv-diff to describe the differences that need to be made to an existing evaluation.