Optimize CI/CD pipeline with Go build cache and artifact caching

[deepakjois]

Summary

This PR improves the CI/CD pipeline performance by enabling Go module caching and adding explicit build artifact caching for the site build process.

Key Changes

• Enable Go build cache: Added cache: true to the actions/setup-go@v6 step to cache Go modules and build cache between workflow runs
• Add build artifact caching: Introduced a new cache step using actions/cache@v4 to cache the build directory, with cache keys based on content, templates, and static file hashes
• Optimize cache invalidation: Cache is invalidated only when relevant source files change (content/, templates/, static/**), reducing unnecessary rebuilds

Benefits

• Faster CI/CD pipeline execution by reusing cached Go modules and build artifacts
• Reduced bandwidth usage and build times on subsequent runs
• Intelligent cache invalidation that only rebuilds when source files actually change