ci: prevent new broken documentation redirects

[realmehmetali]

What changed? Why?

Added a dependency-free redirect validator and PR check for docs/docs.json. It resolves direct pages, multi-hop redirects, wildcard/dynamic routes, external destinations, and cycles.

The current default branch already contains redirect debt tracked in #1426 and #1599. Comparing each PR with its base revision lets CI reject newly broken redirects or page deletions without forcing unrelated contributors to repair the existing backlog.

Notes to reviewers

• This changes no redirect destinations or documentation content.
• A redirect that was already invalid on the base revision remains non-blocking.
• A new invalid redirect, a changed redirect that becomes invalid, a new cycle, or deletion of a redirect target fails CI.
• The workflow follows the repository's existing pinned checkout and runner-hardening actions.

How has it been tested?

• node --test scripts/validate-redirects.test.js — 5/5 passing
• node scripts/validate-redirects.js --base-ref master — passing
• node --check on both JavaScript files
• git diff --check

AI disclosure: Codex assisted implementation and testing. I reviewed the current public repository, public redirect issues, test behavior, and final diff. No private code, data, credentials, or internal product details were used or included.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1