Hugo and Netlify NPM pkgs: update & change to peer; update UG

[chalin]

• Updates Hugo one minor patch version to 0.147.6
• Contributes to [CI/infra] Rework NPM package dependencies #2115
• A step towards addressing the problems detailed in Docsy as an NPM package: publish package, adjust dev deps, and document choices #2172
• Makes Hugo and Netlify-cli peer dependences. Marks Netlify-cli as optional, and hence as of NPM v11, will no longer be installed by default.
• Updates UG to clarify that projects installing Docsy via NPM can use --omit=peer to exclude Hugo and Netlify-cli packages. Or omit the --omit=peer and have the officially supported version of Hugo installed via hugo-extended
• Adjusts UG toc level inclusion
• Preview: https://deploy-preview-2274--docsydocs.netlify.app/docs/get-started/other-options/#option-3-docsy-as-an-npm-package

/cc @LisaFC @emckean

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