[docs] fix broken links

[jameslamb]

Fixes broken links in the project's documentation, detected by the link-checks CI job: https://github.com/microsoft/LightGBM/actions/runs/5948642832/job/16132829498.

URL        `https://www.rpmfind.net/linux/fedora/linux/releases/36/Everything/x86_64/os/Packages/m/mingw64-boost-static-1.75.0-6.fc36.noarch.rpm'
Name       `Prebuilt Boost x86_64'
Parent URL file:///home/runner/work/LightGBM/LightGBM/docs/_build/html/GPU-Windows.html, line 180, col 14
Real URL   https://www.rpmfind.net/linux/fedora/linux/releases/36/Everything/x86_64/os/Packages/m/mingw64-boost-static-1.75.0-6.fc36.noarch.rpm
Result     Error: 404 Not Found

URL        `https://www.rpmfind.net/linux/fedora/linux/releases/36/Everything/x86_64/os/Packages/m/mingw32-boost-static-1.75.0-6.fc36.noarch.rpm'
Name       `Prebuilt Boost i686'
Parent URL file:///home/runner/work/LightGBM/LightGBM/docs/_build/html/GPU-Windows.html, line 180, col 213
Real URL   https://www.rpmfind.net/linux/fedora/linux/releases/36/Everything/x86_64/os/Packages/m/mingw32-boost-static-1.75.0-6.fc36.noarch.rpm
Result     Error: 404 Not Found

URL        `https://github.com/microsoft/SynapseML/blob/master/notebooks/features/lightgbm/LightGBM%20-%20Overview.ipynb'
Name       `this SynapseML example'
Parent URL file:///home/runner/work/LightGBM/LightGBM/docs/_build/html/Parallel-Learning-Guide.html, line 176, col 8
Real URL   https://github.com/microsoft/SynapseML/blob/master/notebooks/features/lightgbm/LightGBM%20-%20Overview.ipynb
Result     Error: 404 Not Found

Looks like our friends at SynapseML did their annual "completely re-arrange all the docs" PR 😂

microsoft/SynapseML#2021

And some older releases of Fedora Boost packages were removed from rpmfind.net.

How I tested this

✅ manually triggered the check-links job targeting this branch: https://github.com/microsoft/LightGBM/actions/runs/5952346174

[github-actions]

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