[Contribution Guide Update] Adding List guidance for sub-resources

[promisinganuj]

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Description

Update the List Resource contributor guide to document how sub-resources should be handled.

This change adds a dedicated Handling List for Sub-resources section to contributing/topics/guide-list-resource.md and clarifies that:

• sub-resources should be listed at the immediate parent scope
• subscription_id and resource_group_name should not be exposed when the API only supports parent-scoped listing
• the immediate parent resource ID should be used as the required input
• the same pattern applies to deeper hierarchies

The new guidance is kept separate from the typed vs untyped implementation sections and includes a short implementation/doc reference example.

Testing

This is a documentation-only change.

PR Checklist

• I have followed the guidelines in our Contributing Documentation.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have checked if my changes close any open issues. If so please include appropriate closing keywords below.
• I have updated/added Documentation as required written in a helpful and kind way to assist users that may be unfamiliar with the resource / data source.
• I have used a meaningful PR title to help maintainers and other users understand this change and help prevent duplicate work.
  For example: “resource_name_here - description of change e.g. adding property new_property_name_here”

This is a (please select all that apply):

• Bug Fix
• New Feature (ie adding a service, resource, or data source)
• Enhancement
• Breaking Change

[gerrytan]

Just a minor suggestion, otherwise this PR looks good. Thx for improving the contrib guide @promisinganuj .