Add a note on Operator releases missing from the OLM catalogs

What does this MR do?

Adds a note beside the startingCSV guidance in doc/installation.md, as agreed with @clemensbeck in #1027 (comment 4026881508). The note says OLM catalogs accept releases only in sequential order, so backport releases for older version lines are not published to them. Operator 3.1.3 and 3.1.4 are backports of this kind, and the test plan shows neither is in the catalogs. A Subscription with a startingCSV for such a release reports ResolutionFailed, and those releases are available only through the Manifest and Helm Chart installation methods.

Test plan

1. On OpenShift 4.22.14, a Subscription to gitlab-operator-kubernetes (community-operators catalog, stable channel) with startingCSV: gitlab-operator-kubernetes.v3.1.3 reports ResolutionFailed: "no operators found with name gitlab-operator-kubernetes.v3.1.3 in channel stable".
2. The same Subscription with v3.1.4 reports the same failure.
3. The same Subscription with v3.2.0 creates an InstallPlan that waits for approval.
4. The operators/gitlab-operator-kubernetes directories in k8s-operatorhub/community-operators (OperatorHub.io), redhat-openshift-ecosystem/community-operators-prod (OpenShift Community Operators), and redhat-openshift-ecosystem/certified-operators (Red Hat Ecosystem Catalog) hold 3.1.2 and 3.2.0 but neither 3.1.3 nor 3.1.4.
5. Each release page lists four manifests, and all eight download.
6. The Helm repository at https://charts.gitlab.io lists chart 3.1.3 and 3.1.4, and the chart archives declare app versions 3.1.3 and 3.1.4.

Related issues

Relates to #1027 (closed)

Author's checklist

• Ensure the branch name starts with docs- or ends with -docs, so only the docs-related CI jobs are included
• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• 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 marked as ready (not a draft) and assigned to a reviewer

If you are only adding documentation, do not add any of the following labels:

• ~"feature"
• ~"frontend"
• ~"backend"
• ~"bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• As soon as you begin reviewing, select Start a review on your first comment so the MR shows Reviewer started review.
• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.

An AI coding assistant wrote this change. The assistant also ran each described step on an OpenShift Local 4.22.14 cluster on a test VM or against the public catalog repositories, release pages, and Helm repository, and ran vale, markdownlint-cli2, and lychee on the changed page from the repository's lint configuration.