Detect missing translation files and report progress in validate_translation.py

[abdeltaehass]

What this does

Improves utils/validate_translation.py so it does more than diff section names between a translation's _toctree.yml and the English source.

• Verifies files exist on disk. Every section referenced in a language's _toctree.yml is now checked for a matching .mdx file. A section that is listed but has no file breaks the doc-builder build (as the README warns), but the old script never caught it. The script now exits non-zero in that case, so it can be used as a CI gate.
• Handles local_fw sections. The previous code read section["local"] directly, silently ignoring framework-specific entries such as chapter8/4_tf. Both PyTorch and TensorFlow variants are now counted.
• Reports progress. Output now shows translated/total sections and a completion percentage instead of dumping the full list of completed sections.
• Fixes the "sesions" typo in the output.

Why

While checking translation status I noticed the script can report a translation as having "no missing sections" even when an entry in _toctree.yml points to a file that doesn't exist — which then fails the build. The on-disk check closes that gap, and the progress summary makes it easier to see how far a translation has come.

Before / After

Before:

Completed sesions:

chapter0/1
chapter1/1
...

Missing sections:

chapter1/2
...

After:

📊 'fr' translation progress: 79/104 sections (76.0%)

📝 Sections not yet translated:

  - chapter12/5
  - ...

When a section is listed in _toctree.yml but its file is missing:

❌ Sections listed in _toctree.yml but missing their .mdx file
   (these break the course build):

  - chapter1/2.mdx

Testing

• Ran against existing translations (ar, fr, en) — progress percentages and missing-section lists are correct.
• Verified the missing-file path flags a listed-but-absent section and exits with status 1.
• python -m black --check utils/validate_translation.py passes.

cc @lewtun @stevhliu

[HuggingFaceDocBuilderDev]

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