[Docs] Resolve .Site.AllPages deprecation

[Junnygram]

I have successfully investigated and resolved the Hugo deprecation warning for .Site.AllPages in github.com/layer5io/academy-example.

Issue Investigation

The deprecation of .Site.AllPages in Hugo v0.156.0 causes build warnings and future breakage. This global accessor is being replaced by the more modern hugo.Sites and .Pages approach, which better handles multi-site and multi-language setups. Our investigation revealed that while the property wasn't directly used in academy-example, it was inherited from the Docsy theme via the offline search index and default sitemap.

Actions Taken

I have implemented the fix by overriding the offending templates and updating configuration:

1. Created assets/json/offline-search-index.json: Overrode the Docsy search index logic with a modern Hugo-compliant version that uses range hugo.Sites to iterate through all pages across all sites.
2. 2. Created layouts/_default/sitemap.xml: Overrode the default sitemap template using the same fixed iteration pattern (verified against a similar fix in the layer5io/docs repository).
3. 3. Updated hugo.yaml: Bumped the minimum Hugo version to 0.156.0 to ensure compatibility with the hugo.Sites accessor and avoid future deprecation warnings.
      Notes for Reviewers
      This PR fixes [Docs] deprecated: .Site.AllPages #108
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