Adds documentation about installing Knative via CI/CD

What does this MR do?

This Docs should only be merged after:

• gitlab-org/cluster-integration/cluster-applications!71 (merged)
• We update https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Managed-Cluster-Applications.gitlab-ci.yml with new release version and knative custom values path

Caveat about the installation of Istio-metrics

Since we had some issues trying to automatically install/uninstall the Istio-metrics so we can count invocations via CI/CD, we're recommending that the user does this step manually instead of relying on the CI/CD to install/uninstall it.

Related issues

#35676 (closed)

Related implementation MR

gitlab-org/cluster-integration/cluster-applications!71 (merged)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.