[L&R][Workflow] Adding instructions regarding how to create Customer Portal accounts

Why is this change being made?

Recently the Customer Portal was updated to facilitate new registrations via the GitLab.com SSO button rather than the previous HTML registration form.

Following this update Support Engineers should direct customers to:

1. Ensure they have a GitLab.com account
2. Ensure that they have signed into the Customer Portal with their GitLab.com account

This is especially relevant to the Add subscription management contact workflow, as the initial SMC will have already been directed to the Customer Portal by their purchase email.

This should be documented as:

• A reference for current SEs who are used to registration being enabled
• Ensuring our workflows are detailed for new SEs

@gitlab-com/support/licensing-subscription

Reference Slack thread

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

---