Update Application Setting API doc to include suggest_pipeline_enabled

What does this MR do and why?

In #337560 (closed), a new application setting is added to allow admins to toggle display of pipeline suggestion banner in a MR without a pipeline.

In this MR

• suggest_pipeline_enabled is added as an optional param to Change application settings API endpoint. This enables implicit validation to ensure that the passed value is of the correct type (Boolean)
• List of settings that can be accessed via API calls docs section is updated to add an entry for suggest_pipeline_enabled setting

MR acceptance checklist

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