Reorganize sharing projects

What does this MR do?

Moves all the information about sharing projects and groups to the Sharing projects and groups page.

• Removes the page Share a project with a group.
• Moves the section Share a group with another group from the Manage groups page.
• Moves the section Add groups to a project from the Members of a project page.
• Moves the sections Prevent group/project sharing from the Group access and permissions page.

Preview of the changed pages:

• https://main-ee-162130.docs.gitlab-review.app/ee/user/project/members/sharing_projects_groups.html
• https://main-ee-162130.docs.gitlab-review.app/ee/user/group/manage.html
• https://main-ee-162130.docs.gitlab-review.app/ee/user/project/members/

Related issues

Combine doc pages about sharing projects (#457992 - closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
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If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
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  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.