Fix broken link in documentation for cert-manager

What does this MR do?

Fix broken link in documentation for cert-manager

- Point the documentation link to the deprecated ArtifactHub page for
  cert-manager to its newer Helm chart installation page maintained by
  the Cloud Native Computing Foundation

Related https://gitlab.com/gitlab-org/charts/gitlab/-/issues/3446

Signed-off-by: Robert Marshall <rmarshall@gitlab.com>

Discovered the broken link while trying to recall how to test CertManager and the new certificates container.

Related issues

Related #3446 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.