Adapt to new template system in Hugo v0.146.0

[deining]

This PR brings adaptions to Hugo's new template system, as described in the official docs.

Please note that with these changes, preview of user guide fails with hugo version 0.145.0 or lower.

[chalin]

Thanks for exploring this. I've created an issue to track the required changes and hold any discussions about it:

• Adapt to new template system in Hugo v0.146.0 #2243

I'll dig in deeper as soon as I can, hopefully today.

[chalin]

@deining - I think that instead I'm going to step through these changes incrementally and update the UG as I go along -- e.g., #2246. /cc @LisaFC @emckean

[chalin]

Thanks for initiating this. Closing since the work is being covered via #2243.

I'm glad to have stepped through this piecemeal. Among other things, the taxonomy-terms layout filename swap might have been more difficult to pinpoint otherwise (#2257).