Building the docs with STYLE=website referenced a stylesheet that long
longer exists on the website, since we changed it to use versioned
references.

To make it less likely for this to happen again, point to a single
stylesheet on the website which will in turn import the required one.
That puts the process entirely within the scope of the website
repository, so next time a version is switched that's the only place
changes have to be made, making them less likely to be missed.

Per (off-list) discussion with Peter Geoghegan and Jonathan Katz.