docs: pin version in links to external docs for [2/3]

[Borda]

What does this PR do?

follow-up for #19584 applied for Fabric

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cc @Borda @carmocca @justusschock @awaelchli

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 fabric: Docs

Check ID	Status
docs-make (fabric, doctest)	success	✅
docs-make (fabric, html)	success	✅

These checks are required after the changes to docs/source-fabric/conf.py, .github/workflows/docs-build.yml, requirements/docs.txt.

🟢 pytorch_lightning: Docs

Check ID	Status
docs-make (pytorch, doctest)	success	✅
docs-make (pytorch, html)	success	✅

These checks are required after the changes to docs/source-pytorch/conf.py, .github/workflows/docs-build.yml, requirements/docs.txt.

🟢 lightning_app: Docs

Check ID	Status
docs-make (app, doctest)	success	✅
docs-make (app, html)	success	✅

These checks are required after the changes to .github/workflows/docs-build.yml, requirements/docs.txt.

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Thank you for your contribution! 💜

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[Borda]

@awaelchli lets merge this to resolve actually blocked update to the latest...

[awaelchli]

Can you open a PR for the update?

[carmocca]

No idea what I'm accepting