Enhance Website Structure for User-Friendly Document Access

[yizha1]

The current website mixed user documents and developer documents together. As a user who only uses Ratify, it is not easy to find user guides, such as tutorials or how-to to use Ratify under different scenarios. For example, the vulnerability report document is under Home/External Plugins/Verifier, which is not intuitive for a user who wants to set up Ratify for verifying vulnerability report. "External Plugins" are advanced concepts for users who want to plug in their own plugins. we may organize the website structure like below:

• Overview
• Quick Start
  • Quick start on Vallina K8s
  • Quick start on AWS
  • Quick start on Azure
  • Quick start on ...
• Users Guides
  • Ensure image authenticity and integrity by verifying Notary Project signatures
  • Ensure image authenticity and integrity by verifying Cosign signatures
  • Ensure images compliant to vulnerability policies
  • Ensure images not containing non-compliant software licenses
• Developer Guides
  • Overview of plugin framework
  • Extend verifiers
  • Extend key management providers
  • Extend authentication
• Adopters
• FAQ

/cc: @FeynmanZhou