Upgrade to Hugo 0.123+ & address breaking changes

[chalin]

Hugo 0.123.0 comes with a rewritten core and some breaking changes -- and some bugs (there are 5 patch release of 0.123 as of the time of writing). But it's not all bad news: the new core promises quicker build times for large sites, among other performance and feature improvements.

But, I'm unable to build Docsy and it's User Guide properly yet using Hugo 0.123, so I'd suggest we wait until that is resolved before we upgrade for OTel.io --> Docsy has been upgraded via google/docsy#1877.

Bugs impacting the website:

• Image block attribute disappears with 0.123.x
  • Embedded image render hook does not honor markdown attributes gohugoio/hugo#12203
• Subpage missing from section's page list when using 0.123.8
  • Wrong .Parent with pages with overlapping names (e.g. sdk and sdk_exporters) gohugoio/hugo#12263

Differences in Hugo behavior:

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

References:

• Hugo 0.123.0, Hugo 0.124.1
• Upgrade to Hugo v0.123.x google/docsy#1873

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Check for diffs using one of:

$ (cd public && git diff -bw --ignore-blank-lines -I 'Hugo 0.12|navbar-dark|theme="dark"|code.jquery|sha512-|<img |var params =|class="text-') | grep ^diff | grep -vE '_print|\.xml$' | head

[chalin]

With #4093, I think that we're good!