chore: remove leftover Docusaurus template code

[vansh-nagar]

Closes #1142.

This removes the publicly reachable Docusaurus sample pages and the unused template components, client module, stale blog/news page implementations, generated translation entries, orphaned footer translation, and checked-in work log.

It also:

• renames DownloadCompoent to DownloadComponent and updates both locale imports
• removes obsolete page exclusions and template comments
• removes unused logo data attributes
• fixes the custom blog sidebar fragment key and replaces its Facebook header with the ASF license header

Validation:

• yarn build (English and Chinese production builds)

The repository's yarn lint command cannot start because markdownlint is not declared in the project dependencies.