Update links in docs

[oleg-nenashev]

For #908

[Goooler]

More links have to be updated?

[oleg-nenashev]

@Goooler Agreed. I patched the link for the GradleUp site, but I will update the rest too.
Javadoc links work well as of now: https://gradleup.com/shadow/api/

[Marcono1234]

Should there be a note on the README (and maybe the User Guide) that this repository previously had a different owner (and maybe link to #908)?
Users might have become familiar with johnrengelman/shadow and https://imperceptiblethoughts.com/shadow/ and might be slightly confused about the different organization now.

[Goooler]

That sounds good!

[Goooler]

This can move on, we land it.

https://github.com/GradleUp/shadow/releases/tag/8.3.0

[oleg-nenashev]

Yup, will get it over the line

[Goooler]

@oleg-nenashev Are you ready to merge this one?

[oleg-nenashev]

I think so

[oleg-nenashev]

Not sure what your policy is for squash managers, but content wise it looks good. Thanks for actually doing most of the work

[Goooler]

Good job!