Clarify which terms to use when referring to "Groups"

In the Update protected.md (!170419 - merged) MR, the following question arose from a comment:

Which terms should be used when referring to Groups?

Options

The following terms are used in the doc/ directory:

Terms used	Description	Results
Root group	Within the code base, we refer to root_groups, which is defined in the Namespace class. They can be inferred to mean the top-level group or parent group with no parent of its own.	46
Top-level group	This has the same meaning (I think) as root group.	301
Top level group	Same as above, just without a hyphen.	18
Parent group	Used to describe a group that contains subgroups.	189
Subgroup	Recommended term to use for groups nested under a parent group.	1147
sub group	This has the same meaning as subgroup, but it's advised not to use this term.	11
Child group	The documentation advises avoiding alternative terms for subgroups, such as child group or low-level group. If this is the case, should we also avoid the opposite, top-level?	5

Additional changes

• /user/workspace/gitlab_agent_configuration.md - "top group" in the diagram
• /user/workspace/index.md - "root group"
• /user/storage_management_automation.md - "sub group"
• /solutions/cloud/aws/gitlab_aws_integration.md - "Top level"