UI updates to new group from at organization level

What does this MR do and why?

Related to #448877 (closed)

The grouptenant scale team is starting to build out an Organization MVC. For more information about what an "Organization" will be see https://docs.gitlab.com/ee/architecture/blueprints/organization/

In !146001 (merged) we added a form to create a group in an organization. This MR makes a few minor UI updates to this form:

• Add SCIM warning alert, same as what is shown on https://gitlab.com/groups/new#create-group-pane
• Add label description with link to docs to Visibility level field
• Use default Visibility level set in the admin area

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the feature flag
   • echo "Feature.enable(:ui_for_organizations)" | gdk rails c
2. Navigate to /-/organizations/default/groups/new

Testing the default visibility level set in admin area

1. Sign in as an admin
2. Go to /admin/application_settings/general
3. Open the Visibility and access controls section
4. Change the Default group visibility setting and save
5. Go to /-/organizations/default/groups/new, the setting should be reflected. It may take a little bit to update due to caching.