Version 3.0 of documentation (unification to v3_ToC) - migration scripts and instructions

[tpmccallum]

This PR will contain migration scripts to assist with the site's unification.

• [x] create_unified_sidebar.py
• [x] check_that_all_pages_are_linked_to_in_sidebar.py
• [x] create_url_vs_markdown_file_vs_toc_label_mapping.py
• [x] move_markdown_files_to_unified_location.py
• [x] create_redirect_syntax_for_manifest.py