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

[hebypaul]

Notes for Reviewers

This PR migrates the exoscale-academy build process to use an npm-managed local Hugo dependency, standardizing the local development experience and mitigating version mismatch issues.

Specific updates include:

• Adding a check-deps target to the Makefile to validate npm and the local hugo binary.
• Rerouting site, serve, build, and clean commands to trigger npm run scripts.
• Adding dev:* scripts to package.json to handle Hugo commands with the correct flags, stripping out unnecessary npx prefixes.
• Updating README.md to remove references to a globally installed Hugo and emphasize the new make setup -> make site workflow.

This PR fixes #407

Signed commits

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[gemini-code-assist]

Code Review

This pull request migrates the project's build and development workflow to use a locally managed Hugo binary via npm, updating the Makefile, README, and package.json scripts accordingly. Feedback on these changes highlights that the theme-update target in the Makefile still incorrectly references the global hugo binary instead of the local one. Additionally, the clean target in the Makefile and the dev:clean script in package.json are redundant and inefficient, and should be refactored or removed to properly clean build artifacts.

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[github-actions]

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