[CI] Migrate to npm-managed local Hugo and standardise Makefile targets

[hebypaul]

Notes for Reviewers
This PR updates the local development environment to use an npm-managed version of Hugo (extended) and standardizes the Makefile targets. This streamlines the setup process by removing the need for contributors to globally install Hugo on their local machines, ensuring version consistency between local environments and CI.

Key Changes:

• Makefile: Added a check-deps target to validate dependencies and rerouted standard commands (site, serve, build, clean) to trigger their respective local npm scripts.
• package.json: Added dev:* scripts (e.g., dev:build, dev:site) to explicitly handle Hugo commands using the local node module.
• Documentation: Updated README.md and CONTRIBUTING.md to remove references to a global Hugo installation and emphasize the unified make setup -> make site workflow.

This PR fixes #208

Signed commits

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[github-actions]

🚀 Preview deployment: https://layer5io.github.io/academy-theme/pr-preview/pr-210/

[gemini-code-assist]

Code Review

This pull request updates the local development workflow to use an NPM-managed local Hugo binary instead of requiring a global Hugo installation. It updates the documentation, introduces a dependency check in the Makefile, and adds helper scripts to package.json. The review feedback suggests removing the redundant dependency check from the clean target, reverting the removal of npx for npm-check-updates to handle missing optional dependencies, and simplifying a repetitive sentence in the contribution guide.

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