[📝 Docs]: Create Overview page for each section of the documentation

[valeriahhdez]

What Docs changes are you proposing?

Following the example of GatsbyJS documentation, we will create an Overview page for every new section of the JSON Schema documentation website, namely:

• Introduction
• Concepts
• Reference
• Specification
• Contributing
• Resources

This Overview will recap the information each section (content bucket) contains so users know what to expect.

Code of Conduct

• I agree to follow this project's Code of Conduct

[benjagm]

@DhairyaMajmudar do you know if there is a standard component doing this:

[DhairyaMajmudar]

Not a specified component but this much line of codes does the kind of same thing

https://github.com/json-schema-org/website/blob/main/components/Sidebar.tsx#L370-L428