Update instructions for adding a member/group

What does this MR do?

Updated the project page as follows:

• Removed the Add a member modal window section, as this is now the default way of adding members and groups. The steps have been incorporated into the Add users to a project and Add groups to a project sections.
• Moved the version history and flag note to the Add users to a project and Add groups to a project sections. Tweaked the content to refer to the modal window specifically.
• Removed the Enable or disable modal window section because the FLAG notes point to the feature flag docs which already explain how to enable/disable flag. Ensured the feature flag name is included in the FLAG notes.

Before: https://docs.gitlab.com/ee/user/project/members/

After: http://main-ee-80618.178.62.207.141.nip.io/ee/user/project/members/

Updated the group page as follows:

• Removed the Share a group modal window as this now the default.
• Updated the instructions for Share a group with another group and added version history and feature flag info.

Before: https://docs.gitlab.com/ee/user/group/#share-a-group-with-another-group

After: http://main-ee-80618.178.62.207.141.nip.io/ee/user/group/#share-a-group-with-another-group

Related issues

https://gitlab.com/gitlab-org/gitlab/-/issues/344003

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
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  • The associated Technical Writer.

If you are 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.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• 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.
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    • 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.
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• Ensure a release milestone is set.