Upgrade Docsy, and Hugo to 0.123+

[chalin]

• Contributes to Upgrade to Hugo 0.123+ & address breaking changes #4064
• Closes Bump hugo-extended from 0.122.0 to 0.123.8 #4126

Here's the diff in generated site files:

$ (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
diff --git a/404.html b/404.html
diff --git a/js/main.js b/js/main.js
diff --git a/refcache.json b/refcache.json
diff --git a/scss/main.css b/scss/main.css
diff --git a/scss/main.css.map b/scss/main.css.map
diff --git a/site/index.html b/site/index.html

Explanation of differences per file:

• 404 page - b/c of this fix copyright text is missing in the page footer in 404.html google/docsy#1872
• main.{js,.css*} - b/c BS was upgraded to 5.3
• refcache: due to jQuery version upgrade (yes, we want to drop jQuery in the future)
• Site index due to build timestamp

[chalin]

Found an issue:

• Image block attribute disappears with 0.123.x

[chalin]

Found another issue:

• Subpage missing from section's page list when using 0.123.8

[chalin]

All issues resolved. Merging.

NOTE: you'll need to run npm install after upgrading the repo.