Update links in SM onboarding email at Duo seat assignment

The following discussion from !164104 (merged) should be addressed:

• @p_cordero started a discussion

  I think Go to GitLab can link to /users/sign_in and perhaps Get Started can link to https://docs.gitlab.com/ee/user/gitlab_duo/index.html ?

Also, these links need to be updated:

• Code suggestions https://docs.gitlab.com/ee/user/gitlab_duo/#code-suggestions
• Code explanations https://docs.gitlab.com/ee/user/gitlab_duo/#code-explanation
• Test generation https://docs.gitlab.com/ee/user/gitlab_duo/#test-generation