fix(docs-infra): add [lang] attribute to index.html of angular.io (#28916)
fix(docs-infra): add [lang] attribute to index.html of angular.io (#28916)

It's the right thing to do, and improves accessiblity.

See: https://dequeuniversity.com/rules/axe/3.1/html-has-lang?application=lighthouse

PR Close #28916

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