Update handbook to include steps to create a group for groups of engineering contractors

Why is this change being made?

I'm currently working with a vendor (a team of engineers), and we found that managing some permissions, rules, and automation can become difficult.

The idea of creating a group in gitlab-com came during a conversation with the contributor success team as a good option to facilitate the automation of rules based on membership of gitlab-org, in other words, validation if an account is a part of not of GitLab team members.

This MR adds a note to vendor onboarding process, so the creation of a group is required.

The idea is to keep contractor groups under this structure:

@gitlab-contractors
    ├── org-a
       ├── team-a1
       ├── project-x
    ├── org-b
    ├── org-c
    └── org-d

How will this help?

This will also facilitate the creation of an exception to avoid contractors being counted towards community contributions or the resident contributor program.

It will also help with access requests, onboarding, and offboarding of contractors when contracts end.

Questions pending to answer

• Who is responsible for the creation of the group? Is this part of an access request?
• For scenarios where a single organization have different teams working on different projects/contracts, is the current proposal still valid?
• How to manage individual contractors?
• Do we use vendor, contractor and temporary service provider indistinctly? Or how should we refer to teams providing services and that are not team members?

Relates to https://gitlab.com/gitlab-com/business-technology/team-member-enablement/issue-tracker/-/issues/2359

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
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• 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
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