Get started: fix npm-based flows for 0.17 Dart Sass, de-dupe prerequisites

[chalin]

• Setup-flow correctness for 0.17: every npm-based get-started and migration flow now installs Dart Sass and runs Hugo through npm scripts, the failure mode a new troubleshooting entry diagnoses (including its warm-cache false-success trap); flow prose, prerequisites, and the docsy-example README now agree. Also corrects the migration guide's invalid hugoVersion.min TOML/JSON and its macOS-failing sed -i.
• Prerequisite ownership: the shared prerequisites page routes its sections by setup option and owns the tool requirements (Windows Bash, Go floor per Hugo); other-options and the setup guides defer to it; restating openings, descriptions, and stale claims trimmed section-wide.
• Publication gate: npm run install:safe lands in docsy-example with the 0.17 wave; publish with the 0.17 deploy/prod fast-forward, not before.
• Preview(s):
  • Start with a prepopulated site
  • Before you begin (prerequisites)
  • Start a site from scratch
  • Other setup options
  • Troubleshooting
  • Migrate to Hugo Modules
  • Update your Docsy Hugo Module

[chalin]

Validation notes for the migration-guide changes (the smoke matrix doesn't exercise this flow):

• The portable sed '/theme = \["docsy/d' config.toml > hugo.toml && rm config.toml replacement was executed on macOS (BSD sed) against a sample config.toml: theme line removed, other lines preserved. The previous sed -i form errors on BSD sed.
• The appended npm commands (sass-embedded pinned install, npm pkg set scripts.hugo=hugo, npm run hugo -- server) are byte-identical to the from-scratch flow's, which the smoke matrix covers on Ubuntu and Windows.
• The Windows tab's findstr line is unchanged; only the same smoke-covered npm commands were appended.
• hugoVersion.min quoting matches the from-scratch page's proven examples of the same shortcode.

[chalin]

Follow-up validation: the migration guide's final Unix TL;DR was executed end-to-end (macOS, hugo-extended 0.164.0, docsy/theme@v0.16.0) on both entry states: a legacy config.toml site and a site already on hugo.toml. Both: theme line removed, config intact, npm run hugo builds cleanly with zero warnings (the step order keeps hugo mod npm pack as the last package.json writer, which is what prevents Hugo's dependencies-out-of-sync warning). The Windows tab is trace-reviewed only.