Updating nav and top level

What does this MR do?

I wanted to make these pages build on each other, so I changed the order slightly. I also opened a related MR to update the nav to match this page.

In these screenshots you can see how in the "after," the page layout matches the left nav order.

The idea is for each page to build on the one before. So you learn how about the things that go on an issue (labels, iterations, milestones) then issues themselves, then how you populate the issues.

Then it moves on to the portfolio management features, which build on issues.

Then finally, after you know what you're doing, you can start using quick actions and build on your core knowledge.

Before	After

Specifically, I made these changes:

• Reordered the groups that were already on the Plan and track work page.
• Updated the left nav to match the order on the page.

I'm not sure where Use Git should go, but left it where it is for now.

Related issues

Related to: #384502 (closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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• ~"backend"
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These labels cause the MR to be added to code verification QA issues.

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