Switch to bundled Book theme and unblank Hugo home pages

[Sendipad]

Motivation

• Remove dependency on a remote Doks theme and use the bundled book theme so the site can render with local templates.
• Fix blank home pages caused by the landing layout which only defines header/menu container blocks and produces no main content when used as a page layout.

Description

• Updated config.toml to set theme = "book" so Hugo uses the local themes/book theme.
• Removed layout: landing from content.en/_index.md and content.ar/_index.md so the home pages render via the normal home/index templates instead of the incomplete landing layout.
• Verified the themes/book layout structure and adjusted front matter to avoid resolving to the empty landing template.

Testing

• Verified file changes with git diff and inspected files with nl/sed; those checks succeeded and show the theme/front-matter updates.
• Attempted to run hugo version but hugo is not installed in the environment so the binary check failed.
• Attempted to install or download Hugo (apt-get and curl) but network/proxy returned HTTP 403 so the runtime build/download could not be completed.
• Attempted a Playwright navigation/screenshot against a local preview server and received ERR_EMPTY_RESPONSE because no local Hugo server was running in this environment.

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Codex Task