[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 and PostCSS on their local machines.

This PR fixes #1093

Signed commits

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[gemini-code-assist]

Code Review

This pull request updates the documentation and build process to manage the extended Hugo version and PostCSS dependencies automatically via npm. It introduces a dependency check in the Makefile and wraps Hugo commands in npm scripts. The review feedback points out an inefficiency in the clean target of the Makefile, where running npm run dev:clean triggers an unnecessary full build before running make site. It is recommended to use the existing npm run clean script instead, which directly removes build artifacts, and consequently remove the redundant dev:clean script from package.json.

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[github-actions]

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[hortison]

Thanks! Please confirm that the makefile here is exactly the same as in GitHub.com/meshery-extensions/meshery-academy and in GitHub.com/meshery/meshery.

[hebypaul]

Thanks! Please confirm that the makefile here is exactly the same as in GitHub.com/meshery-extensions/meshery-academy and in GitHub.com/meshery/meshery.

The core standardized targets (check-deps, site, serve) now use the exact same npm scripts across all three repositories. However, an exact byte-for-byte match isn't possible because each Makefile retains its own project-specific targets, such as the Docker builds in meshery/meshery and theme updates in meshery-academy