📖 Update cert-manager links after migration

[apricote]

What this PR does / why we need it:

cert-manager was migrated from the jetstack github organisation to a new cert-manager github organisation. While we already fixed the bug that this caused in #6050, the documentation was not updated.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): -

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[palnabarun]

/ok-to-test

[sbueringer]

@apricote Thank you!

/lgtm

[fabriziopandini]

Thanks, @apricote for this cleanup, really appreciated it!
@sbueringer what about backporting given that it is mostly documentation and it can help our users?

[fabriziopandini]

/lgtm
/approve

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[sbueringer]

@sbueringer what about backporting given that it is mostly documentation and it can help our users?

+1

/cherry-pick release-1.1

[k8s-infra-cherrypick-robot]

@sbueringer: new pull request created: #6123

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In response to this:

@sbueringer what about backporting given that it is mostly documentation and it can help our users?

+1

/cherry-pick release-1.1

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