Add documentation for add-on specific assessments to developer.yoast.com

[agnieszkaszuba]

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This task is related to another task on adding documentation on Free and Premium assessments.
In this task, we want to add documentation on add-on specific assessments:

• Local SEO assessments
  • LocalStoreLocatorContentAssessment
  • LocalTitleAssessment
  • LocalSlugAssessment
  • LocalSchemaAssessment
• Video SEO assessments
  • Video body assessment
  • Video title assessment
• if applicable, assessments from other add-ons (double check if there are any)

Note that Woo-specific assessments will be covered in another task: Add documentation for e-commerce specific assessments and scoring criteria to developer.yoast.com