WIP|docs: Fix instructions of valid OCP platform types

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PR Overview

This PR updates the cluster validation documentation to fix and clarify instructions for valid OpenShift platform types.

• Updated table structure with three columns: Platform Type, Installation method, and Documentation.
• Added rows for distinct installation methods and documentation links.
• Included a note clarifying when to use the 'None' platform type.

Reviewed Changes

File	Description
docs/guides/cluster-validation/index.md	Updated table format and clarified usage of platform types.

Copilot reviewed 1 out of 1 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

docs/guides/cluster-validation/index.md:101

• [nitpick] Consider rephrasing this note for clarity and consistency. For example, use 'Platform type None should only be used in clusters installed with Assisted Installer using the User-Managed networking mode when required; otherwise, use platform type External.'

*platform type `None` should be used only in clusters installed with Assisted Installer  with `User-managed` networking mode only when it is required, otherwise use options with platform type `External`.

docs/guides/cluster-validation/index.md:99

• [nitpick] The 'External' platform type appears twice with different installation methods. Consider clarifying or differentiating these entries to reduce potential confusion.

| `External`    | Agent Based Installer | [OpenShift Product][abi-external] |