Fix URLs to the install page

[matthiasnwt]

Description

Just a small fix of four URLs which should lead to the install page but resulted in a 404 error.

"https://scikit-image.org/docs/stable/install.html"

changed to

"https://scikit-image.org/docs/stable/user_guide/install.html"

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[matthiasnwt]

As I haven't change any code, failing the tests is probably related to #6994

[mkcor]

Thank you, @matthiasnwt! Yes, CI failures should be unrelated to this PR.
There's this one #7000, which we keep seeing today, but the other one about batch sizes... I'm not sure.