Fix download links now broken in Spyder 6 & fix error w/stylelint

[CAM-Gerlach]

Pull Request

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• Read and followed this repo's Contributing Guidelines
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Description of Changes

Discovered in and blocks PR #377

When Spyder 6 was released earlier today, the existing download links all broke due to the changes to the filenames to make them more consistent, as well as some of the extensions, and the new Spyder website without a separate download section. Therefore, we've fixed these issues as well as adding a new Linux installer button.

Technically this is going in before the Spyder 5 branch gets cut, but as the Spyder 5 links would be broken anyway (we'd have to hardcode them to the latest version) and are of increasingly dubious various for new users now that Spyder 6 is (nominally) stable, and this is blocking other PRs that we want to apply to the Spyder 5 docs as well (i.e. for the translations that we've already done), I'm going and merging this to master before cutting that branch.

Additionally, I've fixed something else that's broken in the meantime and causing the lint check to fail, the stylelint-no-unsupported-browser-features check causing stylelint to crash due to a change in caniuse ( RJWadley/stylelint-no-unsupported-browser-features/issues#299 ), by pinning the latter dependency to a known good version.

[ccordoba12]

Only one comment for you @CAM-Gerlach, the rest looks good to me.

[ccordoba12]

Thanks @CAM-Gerlach!