DOC-2394: Set up tinymce/3 branch for development

[MitchC1999]

Ticket: DOC-2394

Site: Staging branch

Changes:

• Build site with antora
• Fix dev playbook
• [ ] Flatten structure
• [ ] Fix errors
• [ ] Suppress warnings

Pre-checks:

• [x] Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/

Review:

• [ ] Documentation Team Lead has reviewed