Add "Remote SAST Scanning" video to documentation

Add the Getting Started with SAST scanning in VSCode video to the GitLab Remote SAST documentation in order to encourage other developers to provide feedback via this issue.

The video provides information on how to enable and use remote SAST scanning along with examples.

Checklist

• I'm using the latest version of the extension (see the latest version in the right column of this page)
  • Extension version: Put your extension version here
• I'm using the latest VS Code version (find the latest version here)
  • VS Code version: Put your VS Code version here
• I'm using a supported version of GitLab (see README for the supported version)
  • GitLab version: Put your GitLab version here, or say "happens on gitlab.com"