Upgrade to Hugo v0.123.x

[chalin]

Upgrade Hugo to v0.123.3+. This will require some care because I'm noticing some nontrivial changes to the generated User Guide pages.

Related tasks:

• copyright text is missing in the page footer in 404.html #1872
• NPE for $parent.File.UniqueID using Hugo 0.123.1. #1874 --> will continue to track separately, since I can't reproduce the problem yet

Related links:

• Hugo discourse post: Anchor-only markdown links get prepended with index page path in 0.123.x

Tasks

• Upgrade to Hugo 0.123.x #1877
• Update Hugo to 0.123.8, and prettier #1891