Update custom flow overview; reorganize custom flow section in sidenav

[alexisintech]

[!IMPORTANT] 🔎 Previews:

• Overview

Explanation:

While doing a small content audit for the IA project, I made it to the custom flows section of the docs. The overview for custom flows was lacking in clarity/explanation. I also noticed that some of the guides in the sidenav were incorrectly placed.

This PR:

Overview

• adds an introductory paragraph to "How authentication flows work in Clerk" to introduce users to the fact that they should read about these concepts before building custom flows (improving the user learning experience)
• Improves organization of the overview by renaming headings
• updates outdated information, such as required_fields to requiredFields
• adds a "Next steps" section to help guide users. We commonly get feedback tickets on this Overview asking for concrete examples.. The concrete examples exist per custom flow guide, so now this "Next steps" section should help them see that they need to choose their adventure themselves

Manifest

• moved guides in their appropriate category in the sidenav

[github-actions[bot]]

Hey, here’s your docs preview: https://clerk.com/docs/pr/1181