ci/rtfd: building both fast docs on PR

[Borda]

What does this PR do?

building fabric and pytorch docs as previewed in RTFD
continued of #17086

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📚 Documentation preview 📚: https://pytorch-lightning--18738.org.readthedocs.build/en/18738/

cc @Borda

[github-actions]

⚡ Required checks status: All passing 🟢

No groups match the files changed in this PR.

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Thank you for your contribution! 💜

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[awaelchli]

Looks nice!

[carmocca]

💯