Add overview and guidance for access and permissions

What does this MR do and why?

• This adds "overview of access and permissions" with onboarding information.
• Update and reorganize navigation to reflect this: gitlab-org/technical-writing/docs-gitlab-com!651

Closes [User Documentation] Guidance for setting up pe... (#478128 - closed)

Out of scope

• Authentication protocols and configuration

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

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