Improve visibility of custom policies content in Deployment document

3scale documentation users on the Customer Portal (access.redhat.com) search for information about custom APIcast policies. The result of the search query (https://access.redhat.com/search/#/?q=custom%20apicast%20policies) links the user to the top of Chapter 6 in the Deployment Options document, which is the correct chapter, but the subsection that contains the content for custom policies is many, many screens down into the chapter (https://access.redhat.com/documentation/en-us/red_hat_3scale/2.3/html/deployment_options/apicast_policies#creating_custom_apicast_policies).

To increase the visibility of this important content about custom policies, and to provide an easy route from search results to section 6.3 Creating custom APIcast policies, please add links from the top of Chapter 6 to the major subsections (Standard Policies, Custom Policies, Policy Chain). This mini-ToC navigation will help users who arrive at the top of Chapter 6 and want to skip down to the section that covers custom policies.