Add documentation for Static reachability on Beta

Introduction

We should add clear documentation for static reachability on Beta so that users can try this feature out.

Implementation Plan

• Add gitlab:dependency_scanning_component:reachability to gitlab taxonomy page
• Add instructions on how to use Static reachability
  • How to enable it
  • Which languages and package managers are supported
  • Warnings : Mention that when we get a native Python library called we will get a warning when SCA-to-sharif-matcher is called since this library is not part of the SBOM.Fixed by #530425 (closed)
  • Limitations: For example the fact that if a direct dep is in_use then we set in_use all its transitive deps. That we don't support offline instances.
• Add a definition of Reachability to https://docs.gitlab.com/user/application_security/terminology/
• Mention Sr in the vulnerability report documentation.
• Describe the reachability status in the vulnerability details page.
• Include an entry for reachability in risk assessment data

/cc @tkopel @dabeles @hacks4oats