Handbook build warnings: deprecations, settings, imports

Summary

Address warnings to stay safe on upgrades and increase CI/CD build speeds.

Relevant logs and/or screenshots and/or links to examples

https://gitlab.com/gitlab-com/content-sites/handbook/-/jobs/11965733017

$ hugo --enableGitInfo
WARN  deprecated: site config key markup.goldmark.renderHooks.link.enableDefault was deprecated in Hugo 0.148.0 and will be removed in a future release. Use markup.goldmark.renderHooks.link.useEmbedded instead. Set to "never" if previous value was false, or set to "fallback" if previous value was true.
WARN  module with path "gitlab.com/gitlab-com/content-sites/docsy-gitlab" is imported for the same version "v0.3.63" more than once
Start building sites … 
hugo v0.151.0-c70ab27ceb841fc9404eab5d2c985ff7595034b7+extended linux/amd64 BuildDate=2025-10-02T13:30:36Z VendorInfo=hugomods
WARN  WARNING: 5 sidebar entries have been truncated. To avoid this, increase `params.ui.sidebar_menu_truncate` to at least 125 (from 120) in your config file. Section: /handbook/support/workflows

Possible solutions

/cc @gitlab-com/content-sites/handbook-tools

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information