feat(#16): add guide introduction page

[Decipher]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other; Content

Description

• Added /guide page content
• ...
• Resolves Migrate Guide content to Drupal druxt.js#542

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

Screenshots/Media: