Fix VertexAPIs research docs and stale helper references

[obdagli]

Summary

This PR fixes two VertexAPIs documentation problems in the research docs:

1. translate.vertexapis.com docs were conflating the working legacy path family (/language/translate/v2) with the non-working plain root /v2 paths.
2. CHEATSHEET.md and QUICK_START.md referenced helper files that are not present in the repo snapshot.

Changes

• Clarified translate-vertexapis-com.md so it distinguishes:
  • working legacy paths: /language/translate/v2, /language/translate/v2/languages
  • non-working plain root paths: /v2, /v2/detect, /v2/languages
  • working modern paths: v3, v3beta1
• Replaced missing-helper references in vertexapis-research/CHEATSHEET.md with repo-contained commands based on aiplatform_runner.py
• Replaced missing-helper references in vertexapis-research/QUICK_START.md with repo-contained commands based on aiplatform_runner.py

Verification

Live-verified on 2026-03-16 with a working local key:

• POST /language/translate/v2 -> 200
• GET /language/translate/v2/languages -> 200
• POST /v2 -> 404
• GET /v2/languages -> 404
• POST /v2/detect -> 404
• POST /v3/projects/vertex/locations/us-central1:translateText -> 200
• POST /v3beta1/projects/vertex/locations/us-central1:translateText -> 200

Also verified that python3 vertexapis-research/aiplatform_runner.py --help still runs.

Closes #3
Closes #4

[obdagli]

Addressing follow-up review feedback:

• fixed the runnable command paths so the examples now work from the repository root via vertexapis-research/aiplatform_runner.py
• clarified the translate.vertexapis.com report so the original 2026-03-01 snapshot is separated from the later live re-check
• synced the legacy /language/translate/v2* notes across the touched docs, including detect

I also re-verified that:

python3 vertexapis-research/aiplatform_runner.py --help

works from the repo root.