ci: make removed URL check aware of docs/redirects.txt

[rajannpatel]

Description

This PR updates the check-removed-urls.yml GitHub workflow to prevent false positives when a documentation page is intentionally removed or renamed, but a proper redirect has already been configured in docs/redirects.txt.

Previously, the workflow performed a strict text diff on the built HTML output. If a URL disappeared, the check failed immediately—completely unaware that a redirect existed.

Key Changes

• Added workflow_call Inputs: Properly decoupled the base reference variables so this workflow can be cleanly reused by other pipelines.
• Robust Path Normalization: Streamlined the find and sed pipeline to parse built HTML structures safely and output clean, predictable path formats.
• Redirect-Aware Validation: Replaced the brittle Bash loop with an inline Python verification script. It parses docs/redirects.txt using the native csv module and checks all structural markdown possibilities (page.md, page/index.md, or directory-level paths) before flagging a URL as missing.

Testing Instructions

1. If a PR removes a page without adding it to docs/redirects.txt, the workflow should fail and list the unhandled URL.
2. If a PR removes a page and correctly logs the source path in docs/redirects.txt, the workflow should print a green checkmark (✅ Ignored) and pass successfully.

Checklist

• Tested changes against private/public fork contexts
• Handled empty lines and comment blocks inside redirects.txt
• Validated fallback behavior for reusable workflow triggers