Improve validation section of Reference Architectures

What does this MR do?

Improve validation and test results section of Reference Architectures.

From gitlab-com/Product#3825 (comment 851232998), this MR focuses on:

• Update https://docs.gitlab.com/ee/administration/reference_architectures/#validation-and-test-results to include the why we perform the tests we do, how we perform the tests and how to interpret the results.
• Under https://docs.gitlab.com/ee/administration/reference_architectures/#validation-and-test-results, add a link to the handbook page
• Link to https://docs.gitlab.com/ee/administration/reference_architectures/#validation-and-test-results from all reference architectures

Before	after
https://docs.gitlab.com/ee/administration/reference_architectures/	http://main-ee-81538.35.193.151.162.nip.io/ee/administration/reference_architectures/

Related issues

gitlab-com/Product#3825

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