chore: add content validation workflow

[oscarbol09]

What

Adds a content validation workflow that runs on every pull request touching roadmaps/** content files, plus a reusable script scripts/validate-content.ts that can also validate the whole repository on demand (workflow_dispatch).

Validation rules (aligned with contributing.md)

For every changed roadmaps/*/content/*.md file the script checks:

• File name must follow <topic-slug>@<node-id>.md
• First line must be a non-empty # Topic Title heading
• Resource links must follow - [@type@Title](url) with an allowed type (roadmap, official, opensource, article, course, podcast, video, book, feed — reused from allowedOfficialRoadmapTopicResourceType)
• Max 8 resource links per topic (as documented in contributing.md)
• No GeeksforGeeks links (as documented in contributing.md)
• Duplicate node ids — a changed file is rejected if its roadmap-slug/node-id already exists in the repository under a different file name (this is what caused the stale-slug duplicates previously cleaned up)

Existing repo-wide issues are only reported by the explicit --all / workflow_dispatch mode, so existing content never blocks regular PRs.

Verification

• npm run validate:content -- --files=... on known-good files passes (exit 0) and fails (exit 1) on files missing the H1, exceeding 8 links, with invalid types, malformed links, GeeksforGeeks links, or duplicate node ids.
• --all on the current master reports 114 files/issues (missing titles, files with >8 links, malformed links, GeeksforGeeks links, and 58 duplicate node-id collisions).
• npx tsc --noEmit and prettier --check pass.

Notes

• Runs on pull_request (paths: roadmaps/**, scripts/**, the workflow file itself) and workflow_dispatch (full validation).
• Uses the same toolchain as the other workflows (pnpm@v11, Node 24, actions/checkout@v4).